Display device and method for manufacturing the same

ABSTRACT

An object is to provide a display device with excellent display characteristics, where a pixel circuit and a driver circuit provided over one substrate are formed using transistors which have different structures corresponding to characteristics of the respective circuits. The driver circuit portion includes a driver circuit transistor in which a gate electrode layer, a source electrode layer, and a drain electrode layer are formed using a metal film, and a channel layer is formed using an oxide semiconductor. The pixel portion includes a pixel transistor in which a gate electrode layer, a source electrode layer, and a drain electrode layer are formed using an oxide conductor, and a semiconductor layer is formed using an oxide semiconductor. The pixel transistor is formed using a light-transmitting material, and thus, a display device with higher aperture ratio can be manufactured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device including an oxidesemiconductor.

2. Description of the Related Art

In recent years, a technique by which transistors are formed usingsemiconductor thin films formed over a substrate having an insulatingsurface has been attracting attention. The transistors are applied to awide range of electronic devices such as ICs or electro-optical devices,and there is an expectation for immediate development of especiallytransistors to be used as switching elements for image display devices.Various metal oxides are used for a variety of applications. Indiumoxide is a well-known material and is used as a transparent electrodematerial which is necessary for liquid crystal displays and the like.

Some metal oxides have semiconductor characteristics. Examples of suchmetal oxides having semiconductor characteristics include tungstenoxide, tin oxide, indium oxide, zinc oxide, and the like. Transistors inwhich a channel formation region is formed using such metal oxideshaving semiconductor characteristics are known (Patent Documents 1 and2).

Furthermore, the transistors including oxide semiconductors haverelatively high field-effect mobility. Thus, with use of thetransistors, a driver circuit in a display device or the like can beformed.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

In a display device or the like, in the case where a pixel portion (alsoreferred to as a pixel circuit) and a driver circuit portion are formedover one substrate, excellent switching characteristics, e.g., a highon-off ratio are needed for a transistor used for the pixel portion, anda high operation speed is needed for a transistor used for the drivercircuit portion.

In particular, as the pixel density of the display device is increased,writing time of a display image is reduced. Therefore, it is preferablethat the transistor used for the driver circuit operate at high speed.Besides, the increased pixel density causes a decrease in the apertureratio, which is a problem in the pixel portion.

Therefore, an embodiment of the present invention disclosed in thisspecification is a display device which can achieve the above object anda manufacturing method thereof.

One embodiment of the present invention disclosed in this specificationis a display device including a pixel portion and a driver circuitportion over one substrate, where the pixel portion includes: a firsttransistor including a first gate electrode layer, a gate insulatinglayer over the first gate electrode layer, a first source electrodelayer and a first drain electrode layer which are over the gateinsulating layer and partly overlap with the first gate electrode layer,and a first oxide semiconductor layer which is over the gate insulatinglayer and partly overlaps with the first source electrode layer and thefirst drain electrode layer; a first oxide insulating layer over thefirst source electrode layer, the first drain electrode layer, and thefirst oxide semiconductor layer; a connection electrode layer which isover the first oxide insulating layer and electrically connected to thefirst drain electrode layer; a second oxide insulating layer over thefirst oxide insulating layer and the connection electrode layer; aprotective insulating layer over the second oxide insulating layer; anda pixel electrode layer which is over the protective insulating layerand electrically connected to the connection electrode layer, and wherethe driver circuit portion includes: a second transistor including asecond gate electrode layer, the gate insulating layer over the secondgate electrode layer, a second oxide semiconductor layer over the gateinsulating layer, and a second source electrode layer and a second drainelectrode layer which are over the second oxide semiconductor layer andpartly overlap with the second oxide semiconductor layer; a second oxideinsulating layer over the second source electrode layer, the seconddrain electrode layer, and the second oxide semiconductor layer; and theprotective insulating layer over the second oxide semiconductor layer.The first gate electrode layer, the gate insulating layer, the firstoxide semiconductor layer, the first source electrode layer, the firstdrain electrode layer, the first oxide insulating layer, the secondoxide insulating layer, the protective insulating layer, and the pixelelectrode layer each have a light-transmitting property.

Note that in this specification, the ordinal numbers such as “first” and“second” in this specification are used for convenience and do notdenote the order of steps and the stacking order of layers. In addition,the ordinal numbers do not denote particular names which specify theinvention.

The first gate electrode layer and the first source and drain electrodelayers of the first transistor are formed using metal oxides. The secondgate electrode layer and the second source and drain electrode layers ofthe second transistor are formed using metals.

As the metal oxide, indium oxide, an alloy of indium oxide and tinoxide, an alloy of indium oxide and zinc oxide, or zinc oxide can beused.

In addition, in the pixel portion, a light-transmitting planarizationinsulating layer may be formed between the second oxide insulating layerand the protective insulating layer.

Furthermore, a conductive layer may be formed over the protectiveinsulating layer which overlaps with the second oxide semiconductorlayer in the driver circuit portion.

Each of the first oxide insulating layer and the second oxide insulatinglayer may be formed using an inorganic insulating film by a sputteringmethod. For example, silicon oxide, silicon nitride oxide, aluminumoxide, or aluminum oxynitride can be used.

Further, in the second transistor, an oxide conductive layer may beformed between the second oxide semiconductor layer and the secondsource electrode layer and between the second oxide semiconductor layerand the second drain electrode layer. Such a structure enables contactresistance to be reduced, with which a transistor operating at highspeed can be manufactured. Note that the oxide conductive layerpreferably contains as a component zinc oxide but no indium oxide. Suchan oxide conductive layer can be formed using zinc oxide, zinc aluminumoxide, zinc aluminum oxynitride, or gallium zinc oxide.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a display device includingthe steps of: forming a first gate electrode layer in a region to be apixel portion; forming a second gate electrode layer in a region to be adriver circuit portion; forming a gate insulating layer over the firstgate electrode layer and the second gate electrode layer; forming afirst transistor by forming a first source electrode layer and a firstdrain electrode layer over the gate insulating layer to partly overlapwith the first gate electrode layer and forming a first oxidesemiconductor layer over the gate insulating layer to partly overlapwith the first source electrode layer and the first drain electrodelayer; forming a second transistor by forming a second oxidesemiconductor layer over the gate insulating layer to partly overlapwith the second gate electrode layer, forming a first oxide insulatinglayer over the first oxide semiconductor layer, and forming a secondsource electrode layer and a second drain electrode layer over the firstoxide insulating layer to partly overlap with the second oxidesemiconductor layer; forming a connection electrode layer electricallyconnected to the first drain electrode layer; forming a second oxideinsulating layer over the first oxide insulating layer, the second oxidesemiconductor layer, the second source electrode layer, and the seconddrain electrode layer; forming a protective insulating layer over thesecond oxide insulating layer; and forming a pixel electrode layer overthe protective insulating layer in the region to be the pixel portion tobe electrically connected to the connection electrode layer.

In the structure of the above-described manufacturing method, the firstgate electrode layer, the gate insulating layer, the first oxidesemiconductor layer, the first source electrode layer, the first drainelectrode layer, the first oxide insulating layer, the second oxideinsulating layer, the protective insulating layer, and the pixelelectrode layer each have a light-transmitting property.

The oxide insulating layer formed over the first oxide semiconductorlayer and the second oxide semiconductor layer is preferably formedwithout exposure to air after dehydration or dehydrogenation so as toprevent reincorporation of water or hydrogen into the oxidesemiconductor layer.

In this specification, the term “dehydration or dehydrogenation” refersto not only elimination of water or H₂ but also elimination of H, OH, orthe like.

Dehydration or dehydrogenation is heat treatment which is preferablyperformed in an atmosphere of an inert gas such as nitrogen or a raregas (such as argon or helium) at a temperature higher than or equal to400° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 425° C. and lower than or equal to 700° C.

In the case where heat treatment is performed in an atmosphere of aninert gas such as nitrogen or a rare gas (such as argon or helium), theoxide semiconductor layer becomes an oxygen-deficient type by the heattreatment to be a low-resistance oxide semiconductor layer, that is, ann-type (such as n⁻-type) oxide semiconductor layer. Then, formation ofan oxide insulating layer to be in contact with the oxide semiconductorlayer allows the oxide semiconductor layer to be in an oxygen-excessstate; accordingly, a high-resistance oxide semiconductor layer, thatis, an i-type oxide semiconductor layer is provided. As a result, it ispossible to form and provide a display device including a highlyreliable transistor having favorable electrical characteristics.

The oxide semiconductor layer is subjected to dehydration ordehydrogenation under such a heat treatment condition that two peaksindicating discharge of water or at least one peak at around 300° C. isnot detected even if thermal desorption spectroscopy (TDS) is performedat up to 450° C. on the dehydrated or dehydrogenated oxide semiconductorlayer. Therefore, even if TDS is performed at up to 450° C. on atransistor including the dehydrated or dehydrogenated oxidesemiconductor layer, at least the peak of water at around 300° C. is notdetected.

It is important to prevent the dehydrated or dehydrogenated oxidesemiconductor layer from being exposed to air so as to preventreincorporation of water or hydrogen. After the dehydration ordehydrogenation, the oxide semiconductor layer is turned into alow-resistance oxide semiconductor layer, that is, an n-type (n⁻-type,n⁺-type, or the like) oxide semiconductor layer, and then, resistance ofthe oxide semiconductor layer is increased, whereby an i-type oxidesemiconductor layer is formed. When a transistor is formed using such anoxide semiconductor layer, the threshold voltage of the transistor ispositive voltage, so that the transistor has a so-called normally-offcharacteristic. It is preferable for a transistor used in a displaydevice that a channel be formed with a gate threshold voltage that is apositive value and as close to 0 V as possible. As for an active-matrixdisplay device, electric characteristics of a transistor included in acircuit are significant and performance of the display device depends onthe electric characteristics. Especially, the threshold voltage of thetransistor is important. If the threshold voltage of the transistor isnegative, the transistor is in the normally-on state; in other words,current flows between the source electrode and the drain electrode evenwhen the gate voltage is 0 V, and it is difficult to control the circuitincluding the transistor. Further, in some case, when the absolute valueof the threshold voltage is higher, the driving voltage is insufficient,which makes it impossible for a transistor to perform switchingoperation even if the threshold voltage is the positive value. In thecase of an n-channel transistor, it is preferable that a channel beformed and drain current begin to flow after positive gate voltage isapplied. A transistor in which a channel is not formed unless thedriving voltage is increased and a transistor in which a channel isformed and drain current flows even in the case of the negative voltagestate are unsuitable for a transistor used for a circuit.

An atmosphere while the temperature is decreased from the temperature atwhich dehydration or dehydrogenation is performed may be changed into anatmosphere different from an atmosphere at the time of increasingtemperature or an atmosphere of heat treatment. For example, with afurnace which has been used for dehydration or dehydrogenation, coolingcan be performed by filling the furnace with a high-purity oxygen gas, ahigh-purity N₂O gas, or an ultra-dry air (with a dew point of −40° C. orlower, preferably −60° C. or lower) while the oxide semiconductor layeris prevented from being exposed to air.

Since a transistor is easily broken due to static electricity or thelike, a protective circuit for protecting the transistor in the pixelportion is preferably provided over the same substrate as a gate line ora source line. The protective circuit is preferably formed with anon-linear element including an oxide semiconductor layer.

In a display device of one embodiment of the present invention, a drivercircuit portion including a driver circuit transistor and a displayportion including a pixel transistor are formed over one substrate.Thus, the manufacturing cost of the display device can be reduced.

With use of an oxide semiconductor layer subjected to heat treatment fordehydration or dehydrogenation, a transistor with excellent electriccharacteristics can be manufactured. Further, when the transistor forthe pixel circuit is formed using a light-transmitting material, adisplay device having high aperture ratio and excellent displaycharacteristics can be manufactured. Furthermore, in a display device inwhich the pixel circuit and a driver circuit are formed over onesubstrate, transistors included in the pixel circuit and the drivercircuit can be formed to have different structures between the circuitsin order to obtain electric characteristics needed for the respectivecircuits easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are cross-sectional process views illustrating oneembodiment of the present invention.

FIGS. 2A to 2E are cross-sectional process views illustrating oneembodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating one embodiment of thepresent invention.

FIGS. 4A1 and 4A2 and FIGS. 4B1 and 4B2 are cross-sectional views andplan views illustrating one embodiment of the present invention.

FIGS. 5A1 and 5A2 and FIG. 5B are plan views and a cross-sectional viewillustrating one embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating one embodiment of thepresent invention.

FIGS. 7A and 7B are block diagrams each illustrating a display device.

FIG. 8A is a configuration diagram of a signal line driver circuit andFIG. 8B is a timing chart describing operation of the signal line drivercircuit.

FIGS. 9A to 9D are circuit diagrams showing a configuration of a shiftregister.

FIG. 10A is a circuit diagram of a shift register and FIG. 10B is atiming chart describing operation of the shift register.

FIG. 11 is a cross-sectional view illustrating a display device.

FIG. 12 is a plan view illustrating a display device.

FIG. 13 is a plan view illustrating a display device.

FIG. 14 illustrates an equivalent circuit of a display device.

FIG. 15 is a cross-sectional view illustrating a display device.

FIG. 16 is a plan view illustrating a display device.

FIG. 17 is a plan view illustrating a display device.

FIG. 18 illustrates an equivalent circuit of a display device.

FIG. 19 is a cross-sectional view illustrating a display device.

FIG. 20 is a plan view illustrating a display device.

FIG. 21 is a cross-sectional view illustrating a display device.

FIG. 22 is a plan view illustrating a display device.

FIG. 23 is a cross-sectional view illustrating a display device.

FIG. 24 is a plan view illustrating a display device.

FIGS. 25A and 25B each illustrate an electronic appliance.

FIGS. 26A and 26B each illustrate an electronic appliance.

FIG. 27 illustrates an electronic appliance.

FIGS. 28A and 28B are cross-sectional views illustrating one embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.Note that in the drawings of this specification, the identical portionsor portions having a similar function are denoted by the identicalreference numerals, and description thereon may be omitted.

Embodiment 1

In this embodiment, a display device and a manufacturing method thereofwill be described in detail with reference to drawings. FIG. 1Eillustrates an example of a cross-sectional structure of a transistor ina driver circuit, a transistor in a pixel portion, and a contact portionof a gate wiring (gate electrode), which are formed over one substrate.

A transistor 450 has a bottom-gate structure called a channel-etch type,and a transistor 460 has a bottom-gate structure called a bottom-contact(also called an inverted-coplanar) type.

The transistor 460 provided in the pixel includes, over a substrate 400having an insulating surface, a gate electrode layer 451 a, a gateinsulating layer 402, an oxide semiconductor layer 454 including achannel formation region, a source electrode layer 455 a, and a drainelectrode layer 455 b. In addition, an oxide insulating layer 426 whichcovers the transistor 460 and is in contact with an upper surface and aside surface of the oxide semiconductor layer 454 is provided.

Although the transistor 460 provided in the pixel is described as asingle-gate transistor, a multi-gate transistor having a plurality ofchannel formation regions may be used as necessary.

Note that the oxide semiconductor layer 454 has a light-transmittingproperty and be formed to partly overlap with the source electrode layer455 a and the drain electrode layer 455 b. In addition, the oxidesemiconductor layer 454 overlaps with the gate electrode layer 451 awith the gate insulating layer 402 having a light-transmitting propertyinterposed therebetween. The channel formation region of the transistor460 provided in the pixel is a region which is in the oxidesemiconductor layer 454 and positioned between a side surface of thesource electrode layer 455 a and a side surface of the drain electrodelayer 455 b, which faces the side surface of the source electrode layer455 a; that is, a region which is in contact with the gate insulatinglayer 402 and overlaps with the gate electrode layer 451 a.

In order to achieve a display device with a high aperture ratio, thesource electrode layer 455 a and the drain electrode layer 455 b of thetransistor 460 are formed using a light-transmitting conductive film.

In addition, a light-transmitting conductive film is used for the gateelectrode layer 451 a of the transistor 460.

The transistor 450 provided in the driver circuit portion includes, overthe substrate 400 having an insulating surface, a gate electrode layer421 a, the gate insulating layer 402, an oxide semiconductor layer 403,a source electrode layer 425 a, and a drain electrode layer 425 b. Here,the oxide semiconductor layer 403 has at least a channel formationregion 423, a high-resistance source region 424 a, and a high-resistancedrain region 424 b. In addition, over the channel formation region 423,the source electrode layer 425 a, and the drain electrode layer 425 b,an oxide insulating layer 427 and a protective insulating layer 428,each of which has a light-transmitting property, are provided.

A first region 424 c and a second region 424 d in the oxidesemiconductor layer 403, which overlap with the oxide insulating layer426, are in an oxygen-excess state like the channel formation region423, and serve to reduce leakage current and also reduce the parasiticcapacitance. Note that when the oxide insulating layer 426 does notoverlap with the oxide semiconductor layer 403, the first region 424 cand the second region 424 d of the oxide semiconductor layer 403 are notformed.

Hereinafter, a manufacturing process of the transistor 450 and thetransistor 460 over one substrate is described with reference to FIGS.1A to 1E.

First, a metal film is formed over the substrate 400 having aninsulating surface, and then by a first photolithography step and anetching step, the gate electrode layer 421 a and a gate electrode layer421 b are formed. Note that although the gate electrode layer 421 bcorresponds to a gate wiring, it is referred to as the gate electrodelayer for convenience.

Note that a resist mask used in the photolithography step may be formedby an inkjet method. Formation of the resist mask by an inkjet methodneeds no photomask; thus, manufacturing cost can be reduced.

As the metal film used for the gate electrode layers 421 a and 421 b,there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloyincluding the above element, an alloy stacked film including any ofthese elements in combination, and the like.

As the substrate 400, a glass substrate, for example, formed fromaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass can be used. In the case where the temperature of heat treatmentto be performed later is high, a substrate whose strain point is higherthan or equal to 730° C. is preferably used as the substrate 400.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be usedinstead of the glass substrate.

An insulating layer serving as a base film may be provided between thesubstrate 400 and the gate electrode layers 421 a and 421 b. The basefilm has a function of preventing diffusion of an impurity element fromthe substrate 400, and can be formed to have a single-layer or stackedstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

Next, a light-transmitting conductive film is formed to cover the gateelectrode layers 421 a and 421 b, and then by performance of a secondphotolithography step and an etching step, the gate electrode layer 451a and a gate electrode layer 451 b are formed. Note that although thegate electrode layer 451 b corresponds to the gate wiring layer, it isreferred to as the gate electrode layer for convenience. As a materialof the light-transmitting conductive film, a conductive material thattransmits visible light, for example, an In—Sn—O-based metal oxide, anIn—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, anSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, anSn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, anSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—O-basedmetal oxide, an Sn—O-based metal oxide, or a Zn—O-based metal oxide canbe employed. The thickness of the conductive film is selected asappropriate in the range of 50 nm to 300 nm inclusive. In the case ofusing a sputtering method, film formation may be performed using atarget containing SiO₂ at 2 wt. % to 10 wt. % inclusive for the aboveconductive material.

In this embodiment, the gate wiring in the pixel portion is partlyformed using the same metal film as the gate electrode layers 421 a and421 b in order to reduce wiring resistance.

Next, the gate insulating layer 402 is formed over the gate electrodelayers 421 a, 421 b, 451 a, and 451 b.

For the gate insulating layer 402, a light-transmitting insulating filmsuch as a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, or a silicon nitride oxide layer can be used, and suchan insulating layer is formed by a plasma CVD method, a sputteringmethod, or the like. Further, the gate insulating layer 402 is notlimited to a single layer of any of the above insulating film but may bea stacked layer including films different from each other. For example,a silicon oxynitride film can be formed using a deposition gas includingsilane (SiH₄), oxygen, and nitrogen by a plasma CVD method. Thethickness of the gate insulating layer 402 is from 100 nm to 500 nminclusive. In the case of a stacked structure, for example, a first gateinsulating layer with a thickness from 50 nm to 200 nm inclusive isformed and a second gate insulating layer with a thickness from 5 nm to300 nm inclusive are formed over the first gate insulating layer.

In this embodiment, the gate insulating layer 402 is formed usingsilicon oxynitride (SiON (the composition ratio N<O)) by a plasma CVDmethod to have a thickness of 100 nm.

Next, a light-transmitting conductive film is formed over the gateinsulating layer 402, and then by performance of a thirdphotolithography step and an etching step, the source electrode layer455 a and the drain electrode layer 455 b are formed (see FIG. 1A).

For the light-transmitting conductive film, the material similar to thatof the gate electrode layers 451 a and 451 b can be used.

Next, through a fourth photolithography step and an etching step, thegate insulating layer 402 is selectively etched, so that a contact holereaching the gate electrode layer 421 b is formed.

Next, over the gate insulating layer 402, a light-transmitting oxidesemiconductor film is formed to a thickness form 5 nm to 200 nminclusive, preferably from 10 nm to 20 nm inclusive by a sputteringmethod. The preferable thickness is 50 nm or smaller, so that the oxidesemiconductor film can have an amorphous structure even when heattreatment for dehydration or dehydrogenation is performed afterformation of the oxide semiconductor film. Reduction in thickness canprevent the oxide semiconductor film from being crystallized when heattreatment is performed later.

As the oxide semiconductor film, any of the following oxidesemiconductor film can be used: an In—Sn—Ga—Zn—O film that is aquaternary metal oxide; an In—Ga—Zn—O film, an In—Sn—Zn—O film,In—Al—Zn—O film, an Sn—Ga—Zn—O film, an Al—Ga—Zn—O film, or anSn—Al—Zn—O film that is a ternary metal oxide; an In—Zn—O film, anSn—Zn—O film, an Al—Zn—O film, a Zn—Mg—O film, an Sn—Mg—O film, or anIn—Mg—O film that is a binary metal oxide; an In—O film; an Sn—O film; aZn—O film; and the like. In addition, the above oxide semiconductor filmmay contain SiO₂.

As the oxide semiconductor film, a thin film represented by InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like. An oxide semiconductor filmwhose composition formula is represented by InMO₃ (ZnO)_(m) (m>0), whichincludes Ga as M, is referred to as an In—Ga—Zn—O-based oxidesemiconductor, and a thin film of the In—Ga—Zn—O-based oxidesemiconductor is also referred to as an In—Ga—Zn—O-basednon-single-crystal film.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basednon-single-crystal film having a thickness of 15 nm is formed using anIn—Ga—Zn—O-based oxide semiconductor target by a sputtering method.

The In—Ga—Zn—O-based non-single-crystal film can be formed in an oxygenatmosphere (the proportion of oxygen flow: 100%) under conditions wherea target is an In—Ga—Zn—O-based oxide semiconductor(In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio], i.e., In:Ga:Zn=1:1:0.5 [atomicratio]), the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, and the direct current (DC) power is 0.5 kW.Alternatively, a target with a composition ratio of In:Ga:Zn=1:1:1[atomic ratio] or In:Ga:Zn=1:1:2 [atomic ratio] may be used. The fillingfactor of such a target is from 90% to 100% inclusive, preferably 95% to99.9% inclusive. With use of a metal oxide target with high fillingfactor, the deposited oxide semiconductor film has high density.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used inthe case of forming an insulating film, and a DC sputtering method ismainly used in the case of forming a conductive film.

Note that in order to reduce dust generated during deposition andimprove uniformity of the thickness distribution, it is preferable thatsputtering be performed with use of a pulsed direct-current (DC) powersource.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can bedeposited to be stacked in the same chamber, or a plurality of kinds ofmaterials can be deposited by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during depositionto form a thin film of a compound thereof, and a bias sputtering inwhich a voltage is also applied to a substrate during deposition.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust on a surface of the gate insulating layer 402 is preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated. The reverse sputtering refers to a method in whichan RF power source is used for application of voltage to a substrateside in an argon atmosphere so that ionized argon collides with thesubstrate to modify a surface. Note that instead of an argon atmosphere,nitrogen, helium, oxygen or the like may be used.

Before formation of the oxide semiconductor film, heat treatment (athigher than or equal to 400° C. and lower than the strain point of thesubstrate) may be performed in an inert gas atmosphere (e.g., nitrogen,helium, neon, or argon), so that impurities such as hydrogen and water,which are included in the gate insulating layer 402, are removed.

Next, by performance of a fifth photolithography step and an etchingstep, the oxide semiconductor film is processed into island-shaped oxidesemiconductor layers 403 and 453 (see FIG. 1B). A resist mask used forformation of the island-shaped oxide semiconductor layers 403 and 453may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

Note that in this embodiment, formation of the contact hole reaching thegate electrode layer 421 b is conducted in the fourth photolithographystep and the etching step where the gate insulating layer is selectivelyetched, before formation of the oxide semiconductor film. However,formation of the contact hole may be conducted after formation of theabove-described island-shaped oxide semiconductor layers 403 and 435. Inthat case, reverse sputtering is preferably performed so that resistresidue on the surface of the oxide semiconductor layers 403 and 453 andthe gate insulating layer 402 are removed.

Further alternatively, the contact hole reaching the gate electrodelayer 421 b may be formed after formation of the oxide semiconductorfilm over the gate insulating layer. In that case, after formation ofthe contact hole, the oxide semiconductor film is selectively etched tobe processed into the island-shaped oxide semiconductor layers 403 and453.

Next, the oxide semiconductor layers 403 and 453 are dehydrated anddehydrogenated. First heat treatment for dehydration or dehydrogenationis performed at a temperature higher than or equal to 400° C. and lowerthan the strain point of the substrate, preferably, 425° C. or higher.Note that in the case of the temperature that is 425° C. or higher, theheat treatment time may be one hour or shorter, whereas in the case ofthe temperature lower than 425° C., the heat treatment time is longerthan one hour.

Here, the substrate is put in an electric furnace which is one of heattreatment apparatuses and heat treatment is performed on the oxidesemiconductor layers 403 and 453 in a nitrogen atmosphere. In thisembodiment, one furnace used in heat treatment is continuously used upto the time when the temperature is lowered from the heat temperature Tat which dehydration or dehydrogenation of the oxide semiconductorlayers 403 and 453 is performed to the temperature which is enough toprevent reincorporation of water. Specifically, slow cooling isperformed in a nitrogen atmosphere up to the time when the temperaturebecomes lower than the heat temperature T by 100° C. or more. Withoutbeing limited to a nitrogen atmosphere, dehydration or dehydrogenationmay be performed in a rare gas atmosphere, such as helium, neon, orargon.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Here, it is preferable that the purity ofnitrogen or a rare gas such as helium, neon, or argon which isintroduced to the heat treatment apparatus be 6N (99.9999%) or more,more preferably 7N (99.99999%) or more.

The oxide semiconductor layers 403 and 453 are each crystallized by thefirst heat treatment to be a microcrystalline film or a polycrystallinefilm in some cases. Further, by the first heat treatment, the oxidesemiconductor layers 403 and 453 each become an oxygen-deficient oxidesemiconductor layer and the carrier concentration is increased to higherthan or equal to 1×10¹⁸/cm³; that is, resistance of the oxidesemiconductor layers 403 and 453 is reduced. In addition, the gateelectrode layers 451 a and 451 b are each crystallized by the first heattreatment to be a microcrystalline film or a polycrystalline film insome cases. For example, in the case where an indium oxide-tin oxidealloy film is used for the gate electrode layers 451 a and 451 b, thegate electrode layers are easily crystallized by the first heattreatment performed at 450° C. for one hour. However, in the case wherean indium oxide-tin oxide alloy film containing silicon oxide is usedfor the gate electrode layers, crystallization is not easily conducted.

The first heat treatment for the oxide semiconductor layer can beperformed before the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layers. In that case, the fifthphotolithography step is performed after the first heat treatment.

Next, an oxide insulating layer having a light-transmitting property isformed by a sputtering method over the gate insulating layer 402 and theoxide semiconductor layers 403 and 453. Then, a resist mask is formed bya sixth photolithography step, etching is selectively performed to formthe oxide insulating layer 426 by an etching step, and then, the resistmask is removed. At this stage, the periphery and side surface of theoxide semiconductor layer 403 and the periphery and side surface of theoxide semiconductor layer 453 overlap with the oxide insulating layer426. By the sixth photolithography step and the etching step, a contacthole reaching the gate electrode layer 421 b and a contact hole reachingthe drain electrode layer 455 b are also formed (see FIG. 1C).

The oxide insulating layer 426 can be formed to a thickness at least 1nm or more by a method, as appropriate, which is a method with whichimpurities such as water, hydrogen, and the like are prevented fromentering the oxide insulating layer. In this embodiment, the oxideinsulating layer 426 is formed from a silicon oxide film by a sputteringmethod.

The substrate temperature in film formation may be equal to or higherthan room temperature and equal to or lower than 300° C. The substratetemperature in this embodiment is 100° C. The formation of the siliconoxide film by a sputtering method can be performed in a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or an atmosphere ofa rare gas (typically, argon) and oxygen.

Moreover, a silicon oxide target or a silicon target can be used as atarget. For example, the silicon oxide film can be formed using asilicon target in an atmosphere of oxygen and a rare gas by a sputteringmethod. The oxide insulating layer formed to be in contact with thelow-resistance oxide semiconductor layers 403 and 453, which includes animpurity such as moisture, a hydrogen ion, or OH⁻ as little as possible,is preferably formed from an inorganic insulating film which can blockentry of such an impurity from the outside. As a typical inorganicinsulating film, a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, an aluminum oxynitride film or the like can beused.

In this embodiment, film formation is performed by a pulsed DCsputtering method in an oxygen atmosphere (the proportion of oxygenflow: 100%) under conditions where a target is a columnarpolycrystalline silicon target doped with boron (resistivity: 0.01 Ωcm,purity: 6N), the distance between the substrate and the target (T-Sdistance) is 89 mm, the pressure is 0.4 P, and the direct current (DC)power is 6 kW. The thickness is 300 nm.

Next, a metal film is formed over the gate insulating layer 402, theoxide insulating layer 426, and the oxide semiconductor layers 403 and453. Then, a resist mask is formed by a seventh photolithography step,and by an etching step, the source electrode layer 425 a and the drainelectrode layer 425 b are formed. Further, a connection electrode layer429 electrically connected to the gate electrode layer 421 b and aconnection electrode layer 452 electrically connected to the drainelectrode layer 455 b are also formed.

The metal film can be formed by a sputtering method, a vacuumevaporation method (e.g., an electron beam evaporation method), an arcdischarge ion plating method, or a spray method. The metal film isformed using an element selected from Ti, Mo, W, Al, Cr, Cu, and Ta, analloy containing any of the above elements as its component, an alloyfilm containing these elements in combination, or the like. In addition,the metal film is not limited to a single layer of the above element butmay be a stacked layer including different elements. In this embodiment,the metal film has a three-layer structure of a titanium film (with athickness of 100 nm), an aluminum film (with a thickness of 200 nm), anda titanium film (with a thickness of 100 nm). Instead of a titaniumfilm, a titanium nitride film may be used.

In the etching step performed after the seventh photolithography step,the metal film which is over and in contact with the oxide semiconductorlayers 403 and 453 needs to be selectively removed. In such a case, withuse of an alkaline etchant (e.g., an ammonia peroxide mixture (hydrogenperoxide water at 31 wt %:ammonia water at 28 wt %:water=5:2:2)) or thelike, the metal film can be selectively removed, so that the oxidesemiconductor layers 403 and 453 formed using an In—Ga—Zn—O-based oxidesemiconductor can be left.

Note that the resist mask used for formation of the source electrodelayer 425 a and the drain electrode layer 425 b may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

Next, the oxide insulating layer 427 having a light-transmittingproperty is formed over the oxide insulating layer 426, the sourceelectrode layer 425 a, the drain electrode layer 425 b, the connectionelectrode layer 429, and the connection electrode layer 452 (see FIG.1D). As the oxide insulating layer 427, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,or the like is used. In this embodiment, the oxide insulating layer 427is formed using a silicon oxide film by a sputtering method.

Next, second heat treatment is performed at a temperature from 200° C.to 400° C. inclusive, preferably 250° C. to 350° C. inclusive in anatmosphere of an inert gas such as a nitrogen gas. For example, thesecond heat treatment in a nitrogen atmosphere at 250° C. is performedfor one hour.

The second heat treatment is performed in such a condition that a partof the oxide semiconductor layer 403 is in contact with the oxideinsulating layer 427 and the oxide semiconductor layer 453 is in contactwith the oxide insulating layer 426. Therefore, the oxide semiconductorlayers 403 and 453, with each of which resistance is reduced by thefirst heat treatment, are supplied with oxygen from the oxide insulatinglayers 427 and 426 to be in an oxygen-excess state, so thathigh-resistance (i-type) oxide semiconductor layers are formed.

Note that in the case where the thickness of the oxide semiconductorlayer 403 is smaller than 15 nm, in regions which are included in theoxide semiconductor layer 403 and overlap with the source electrodelayer 425 a and the drain electrode layer 425 b formed from the metalfilm, oxygen easily moves to the metal film side, and the regions areentirely turned into n-type regions. Alternatively, in the case wherethe thickness of the oxide semiconductor layer 403 is from 15 nm to 50nm inclusive, the vicinity of an interface between the metal film andthe regions becomes to be in an n-type state, but regions in the oxidesemiconductor layer 403 below the n-type region becomes an i-type orn⁻-type region.

Note that although the second heat treatment is performed afterformation of the silicon oxide film in this embodiment, the heattreatment can be performed at any time as long as it is performed afterformation of the silicon oxide film and the timing of the heat treatmentis not limited to a timing immediately after formation of the siliconoxide film.

Next, the protective insulating layer 428 having a light-transmittingproperty is formed over the oxide insulating layer 427 (see FIG. 1E). Asthe protective insulating layer 428, a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, or the like is used. Inthis embodiment, the protective insulating layer 428 is formed using asilicon nitride film by an RF sputtering method.

Although not illustrated, a light-transmitting planarization insulatinglayer may be provided between the oxide insulating layer 427 and theprotective insulating layer 428 in the pixel portion. The planarizationinsulating layer can be formed using an organic material having heatresistance such as an acrylic-based resin, polyimide, abenzocyclobutene-based resin, polyamide, or an epoxy-based resin. Inaddition to such organic materials, a low-dielectric constant material(a low-k material), a siloxane-based resin, PSG (phosphosilicate glass),BPSG (borophosphosilicate glass), or the like can be used. Note that theplanarization insulating layer may be formed by stacking a plurality ofinsulating films formed of these materials.

Through the above steps, the channel-etch type transistor 450 and thebottom-contact type transistor 460 can be formed over one substrate.Note that since in the bottom-contact type transistor 460, componentsother than the connection electrode layer 452 are formed usinglight-transmitting materials, the aperture ratio of the transistor 460can be increased.

A channel-etch type transistor like the transistor 450 can be formed tohave a small channel length easily, which is suitable for formation of atransistor like that in a driver circuit, which needs high speedoperation. In other words, a display device including channel-etch typetransistors can operate at higher speed than a display device in whichplural circuits over one substrate are all formed using bottom-contacttype transistors like the transistor 460.

Further, a pixel electrode which is necessary for the display device isprovided over the protective insulating layer 428 in the pixel portionand electrically connected to the drain electrode layer of thetransistor 460. Here, the pixel electrode may be connected to theconnection electrode layer 452. Note that, for the pixel electrode, alight-transmitting conductive film can be used, which is similar to thatused for the gate electrode layers 451 a and 451 b, the source electrodelayer 455 a, and the drain electrode layer 455 b.

According to one embodiment of the present invention, in a displaydevice where a driver circuit and a pixel circuit are provided over onesubstrate, transistors included in the circuits are formed to havestructures different between the driver circuit and the pixel circuit inorder to obtain electrical characteristics necessary for the respectivecircuits. As in this embodiment, when the channel-etch type transistor450 is used for the driver circuit and the bottom-contact transistor 460is used for the pixel circuit, a display device having excellent displaycharacteristics can be manufactured.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

In this embodiment, an example of a transistor whose manufacturingprocess is partly different from that of Embodiment 1 will be describedwith reference to FIGS. 2A to 2E. FIGS. 2A to 2E are the same as FIGS.1A to 1E except that there is a difference in part of the process.Therefore, the same portions are denoted by the same reference numerals,and detailed description of the same portions is omitted.

First, in accordance with Embodiment 1, the gate electrode layers 421 aand 451 a and the gate insulating layer 402 are formed over thesubstrate, and then the source electrode layer 455 a and the drainelectrode layer 455 b are formed to partly overlap with the gateelectrode layer 451 a with the gate insulating layer 402 interposedtherebetween. Then, an oxide semiconductor film is formed over the gateinsulating layer 402, the source electrode layer 455 a, and the drainelectrode layer 455 b.

Then, the oxide semiconductor film is subjected to dehydration ordehydrogenation. First heat treatment for dehydration or dehydrogenationis performed at a temperature higher than or equal to 400° C. and lowerthan the strain point of the substrate, preferably, 425° C. or higher.Note that in the case of the temperature that is 425° C. or higher, theheat treatment time may be one hour or shorter, whereas in the case ofthe temperature lower than 425° C., the heat treatment time is longerthan one hour. Here, the substrate is put in an electric furnace whichis one of heat treatment apparatuses and the oxide semiconductor film issubjected to heat treatment in a nitrogen atmosphere. Then, the oxidesemiconductor film is not exposed to air, which prevents reincorporationof water and hydrogen into the oxide semiconductor film. After that, ahigh-purity oxygen gas, a high-purity N₂O gas, or ultra-dry air (havinga dew point of −40° C. or lower, preferably −60° C. or lower) isintroduced to the same furnace and cooling is performed. It ispreferable that water, hydrogen, and the like be not contained in theoxygen gas or the N₂O gas. Alternatively, the oxygen gas or the N₂O gas,which is introduced into the heat treatment apparatus, preferably haspurity of 6N (99.9999%) or higher, further preferably purity of 7N(99.99999%) or higher. In other words, an impurity concentration in theoxygen gas or the N₂O gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower.

In addition, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at a temperature from200° C. to 400° C. inclusive, preferably from 200° C. to 300° C.inclusive, in an oxygen gas atmosphere or an N₂O gas atmosphere.

Through the above step, the entire oxide semiconductor film is in anoxygen-excess state, and the oxide semiconductor film can have highresistance: that is, an i-type oxide semiconductor film can be obtained.Note that in this embodiment, an example in which the first heattreatment is performed immediately after formation of the oxidesemiconductor film is described; however, the timing when the first heattreatment is performed is not particularly limited as long as it isperformed after formation of the oxide semiconductor film.

Next, a resist mask is formed by a photolithography step, and by anetching step, the oxide semiconductor film and the gate insulating layer402 are selectively etched, so that a contact hole reaching the gateelectrode layer 421 b is formed. After that, the resist mask is removed(see FIG. 2A).

Next, a resist mask is formed by a photolithography step, and by anetching step, the oxide semiconductor film is selectively etched to beprocessed into an island shape. By removing the resist mask, oxidesemiconductor layers 404 and 405 are provided over the gate insulatinglayer 402 (see FIG. 2B).

Next, over the gate insulating layer 402 and the oxide semiconductorlayers 404 and 405, an oxide insulating layer is formed by a sputteringmethod. Then, a resist mask is formed by a photolithography step, and byan etching step, the oxide insulating layer 426 is formed. After that,the resist mask is removed. At this stage, a region where the oxideinsulating layer 426 overlaps with the oxide semiconductor layer 404 anda region where the oxide insulating layer 426 overlaps with the oxidesemiconductor layer 405 are formed. In addition, in this step, a contacthole reaching the gate electrode layer 421 b and a contact hole reachingthe drain electrode layer 455 b are also formed (see FIG. 2C).

The oxide insulating layer is preferably formed from an inorganicinsulating film which includes impurity as little as possible, such asmoisture, a hydrogen ion, or OH⁻ and blocks entry of these from theoutside. Typically, a silicon oxide film, a silicon nitride oxide film,an aluminum oxide film, an aluminum oxynitride film or the like can beused.

Next, a stack of an oxide conductive film and a metal film is formedover the gate insulating layer 402, the oxide insulating layer 426, andthe oxide semiconductor layers 404 and 405. With use of a sputteringmethod, the oxide conductive film and the metal film can be successivelyformed to have a stacked structure without being exposed to air.

The oxide conductive film preferably contains zinc oxide as itscomponent but no indium oxide. Such an oxide conductive film can beformed using zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride,gallium zinc oxide, or the like. In this embodiment, a zinc oxide filmis used.

Further, the metal film can be formed using an element selected from Ti,Mo, W, Al, Cr, Cu, and Ta, an alloy including the above element, analloy including any of these elements in combination, or the like. Themetal film is not limited to a single layer of the above element but maybe a stacked layer including elements different from each other. In thisembodiment, a stacked layer including three layers of a molybdenum film,an aluminum film, and a molybdenum film.

Next, a resist mask is formed by a photolithography step, and by anetching step, the metal film is selectively etched, so that a sourceelectrode layer 445 a, a drain electrode layer 445 b, a connectionelectrode layer 449, and a connection electrode layer 442 are formed.After that, the resist mask is removed.

A resist stripper used for removing the resist mask is an alkalinesolution, and in the case where the resist stripper is used, the zincoxide film is selectively etched with use of the above electrode layersas masks. Thus, an oxide conductive layer 446 a in contact with thesource electrode layer 445 a and an oxide conductive layer 446 b incontact with the drain electrode layer 445 b are provided.

The etching rate is different between the oxide semiconductor layer andthe oxide conductive layer, and therefore, the oxide conductive layerwhich is on and in contact with the oxide semiconductor layer can beremoved by controlling the time of period.

Alternatively, after the metal film is selectively etched, the resistmask is removed by oxygen ashing treatment. Then, the zinc oxide filmmay be selectively etched by using as masks, the source electrode layer445 a, the drain electrode layer 445 b, the connection electrode layer449, and the connection electrode layer 442.

The oxide conductive layer 446 a provided between the source electrodelayer 445 a and the oxide semiconductor layer 404 serves as a sourceregion, and the oxide conductive layer 446 b provided between the drainelectrode layer 445 b and the oxide semiconductor layer 404 serves as adrain region. Providing the oxide conductive layer 446 a and the oxideconductive layer 446 b allows reduction of contact resistance betweenthe oxide semiconductor layer 404 and the source and drain electrodelayers 445 a and 445 b. As a result, a transistor in which resistance ofthe current path is reduced can operate at high speed, and frequencycharacteristics of a periphery circuit (driver circuit) can be improved.

Molybdenum is a material whose contact resistance with an oxidesemiconductor is relatively high. This is because molybdenum is lesslikely to be oxidized and has a weaker effect of extracting oxygen fromthe oxide semiconductor layer as compared to titanium, and a contactinterface between molybdenum and the oxide semiconductor layer is notchanged to have an n-type conductivity. Providing an oxide conductivelayer between the oxide semiconductor layer and the metal electrodelayer is significantly effective in reduction of contact resistance evenin the case of using molybdenum.

Further, in the same step, an oxide conductive layer 448 in contact withthe connection electrode layer 449 and an oxide conductive layer 447 incontact with the connection electrode layer 442 are formed (see FIG.2D).

Next, in order to reduce variation in electric characteristics of thetransistors, second heat treatment may be performed in an inert gasatmosphere such as a nitrogen gas atmosphere. The second heat treatmentis preferably performed at equal to or higher than 150° C. and lowerthan 350° C. For example, heat treatment is performed at 250° C. in anitrogen atmosphere for one hour.

Note that the second heat treatment causes entry or diffusion of oxygenin the oxide semiconductor layers 404 and 454. By entry or diffusion ofoxygen in the oxide semiconductor layers 404 and 454, resistance ofchannel formation regions can be increased; that is, i-type channelformation regions can be obtained. Therefore, transistors havingelectric characteristics of normally off can be obtained. In addition,the second heat treatment causes crystallization of the oxide conductivelayers 446 a, 446 b, 447, and 448, and thus conductivity can beincreased.

Next, the oxide insulating layer 427 and the protective insulating layer428 are formed over the oxide insulating layer 426, the source electrodelayer 445 a, and the drain electrode layer 445 b (see FIG. 2E). Theoxide insulating layer 427 and the protective insulating layer 428 canbe formed using a material and method similar to those of Embodiment 1.

Through the above steps, a transistor 440 and the transistor 460 can beformed over one substrate.

The transistor 440 provided in the driver circuit portion includes overthe substrate 400 having an insulating surface, the gate electrode layer421 a, the gate insulating layer 402, the oxide semiconductor layer 404,the oxide conductive layers 446 a and 446 b, the source electrode layer445 a, and the drain electrode layer 445 b. Here, the oxidesemiconductor layer 404 has at least a channel formation region 443, ahigh-resistance source region 444 a, and a high-resistance drain region444 b. The oxide insulating layer 427 and the protective insulatinglayer 428 are provided over the channel formation region 443, the sourceelectrode layer 445 a, and the drain electrode layer 445 b.

The oxide conductive layer 446 a serving as a source region is providedbetween the high-resistance source region 444 a and the source electrodelayer 445 a, and the oxide conductive layer 446 b serving as a drainregion is provided between the high-resistance drain region 444 b andthe drain electrode layer 445 b; thus, contact resistance is reduced.

Further, a first region 444 c and a second region 444 d which are in theoxide semiconductor layer 404 and overlap with the oxide insulatinglayer 426 are in an oxygen-excess state like the channel formationregion 443, and serves to reduce leakage current and also reduce theparasitic capacitance. Note that in the case where the oxide insulatinglayer 426 does not overlap with the oxide semiconductor layer 404,neither the first region 444 c nor the second region 444 d are formed inthe oxide semiconductor layer 404.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment, an example in which a liquid crystal display deviceis manufactured with the active-matrix substrate described in Embodiment1 or 2 will be described.

FIG. 3 illustrates an example of a cross-sectional structure of anactive-matrix substrate.

Embodiments 1 and 2 show the example in which the transistor for thedriver circuit portion, the transistor for the pixel portion, and thegate wiring (gate electrode) contact portion are formed over onesubstrate. In this embodiment, in addition to the above components, astorage capacitor and an intersection of the gate wiring and the sourcewiring are illustrated and described.

The capacitor, the gate wiring, and the source wiring can be formed inthe same manufacturing steps as those in Embodiment 1 or 2, without anincrease in the number of photomasks and an increase in the number ofsteps. Further, in a portion serving as a display region in a pixelportion, the gate wiring, the source wiring, and a capacitor wiringlayer are all formed using light-transmitting conductive films, whichresults in high aperture ratio. Furthermore, a metal wiring can be usedfor the source wiring layer other than the display region in order toreduce the wiring resistance.

In FIG. 3, the transistor 450 is a transistor provided in a drivercircuit portion, and the transistor 460 is a transistor provided in apixel portion, which is electrically connected to a pixel electrodelayer 457.

In this embodiment, the transistor 460 formed over the substrate 400 hasthe same structure as the transistor 460 in Embodiment 1 or 2.

A capacitor wiring layer 430, which is formed using the samelight-transmitting conductive material in the same step as the gateelectrode layer 451 a of the transistor 460, overlaps with a capacitorelectrode 431 with a gate insulating layer 402 serving as a dielectricinterposed therebetween, thereby forming the storage capacitor. Notethat the capacitor electrode 431 is formed using the samelight-transmitting material in the same process as the source electrodelayer 455 a and the drain electrode layer 455 b of the transistor 460.Since the storage capacitor as well as the transistor 460 has alight-transmitting property, the aperture ratio can be increased.

The light-transmitting property of the storage capacitor is important inincreasing the aperture ratio. For small liquid crystal display panelsof 10 inches or smaller in particular, high aperture ratio can beachieved even when the size of pixels is decreased. Moreover, wideviewing angle is realized by using a light-transmitting film forcomponents in the transistor 460 and the storage capacitor, so that highaperture ratio can be achieved even when one pixel is divided into aplurality of subpixels. For example, when one pixel includes two to foursubpixels and a storage capacitor, the storage capacitors have lighttransmitting properties as well as the transistors, so that the apertureratio can be increased.

Note that the storage capacitor is provided below the pixel electrodelayer 457, and the capacitor electrode 431 is electrically connected tothe pixel electrode layer 457.

An example in which the storage capacitor is formed with the capacitorelectrode 431, the gate insulating layer 402, and the capacitor wiringlayer 430 is described in this embodiment, but there is no particularlimitation on the structure forming the storage capacitor. For example,instead of providing the capacitor wiring layer, part of the gate wiringin an adjacent pixel may be used as the capacitor wiring layer.Alternatively, besides the gate insulating layer, an insulating layerincluded in the pixel portion, such as a protective insulating layer ora planarization insulating layer, may be used as a dielectric.

A plurality of gate wiring layers, source wiring layers, and capacitorwiring layers are provided depending on the pixel density. In theterminal portion, a plurality of first terminal electrodes at the samepotential as the gate wiring, a plurality of second terminal electrodesat the same potential as the source wiring, a plurality of thirdterminal electrodes at the same potential as the capacitor wiring layer,and the like are arranged. There is no particular limitation on thenumber of each of the terminal electrodes, and the number of theterminal electrodes may be determined by a practitioner as appropriate.

In the gate wiring contact portion, the gate electrode layer 421 b canbe formed using a low-resistance metal material. The gate electrodelayer 421 b is electrically connected to the connection electrode layer429 through a contact hole reaching the gate wiring.

The gate electrode layer of the transistor 450 in the driver circuitportion may be electrically connected to a conductive layer 417 providedabove the oxide semiconductor layer.

In a wiring intersection portion, in order to reduce parasiticcapacitance, the gate insulating layer 402 and the oxide insulatinglayer 426 are stacked between a gate wiring layer 421 c and a sourcewiring layer 422 as illustrated in FIG. 3. Note that although the gatewiring layer 421 c is formed from a metal film in the case of FIG. 3,the gate wiring layer 421 c can be formed using the samelight-transmitting conductive film as the gate electrode layer 451 a ofthe transistor 460.

Further, in the case of manufacturing an active-matrix liquid crystaldisplay device, an active-matrix substrate and a counter substrateprovided with a counter electrode are bonded to each other with a liquidcrystal layer therebetween. A common electrode electrically connected tothe counter electrode on the counter substrate is provided over theactive-matrix substrate, and a fourth terminal electrode electricallyconnected to the common electrode is provided in the terminal portion.The fourth terminal electrode is used for setting the common electrodeto a fixed potential such as GND or 0 V. The fourth terminal electrodecan be formed using the same light-transmitting material as the pixelelectrode layer 457.

When the gate electrode, the source electrode, the drain electrode, thepixel electrode, another electrode, and some wiring layers are formedusing the same material, the same sputtering target and the samemanufacture apparatus can be used. In addition, the material cost andthe cost relating to an etchant or an etching gas used in etching can bereduced, which results in reduction in the manufacturing cost.

In the case where a photosensitive resin material is used for theplanarization insulating layer 456 in the structure of FIG. 3, the stepof formation of a resist mask can be omitted.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, an example of a structure of a terminal portionprovided over the same substrate as the transistor will be describedwith reference to FIGS. 4A1 to 4B2. Note that in FIGS. 4A1 to 4B2,components common to FIG. 3 maintain the same reference numerals.

FIGS. 4A1 and 4A2 respectively illustrate a cross-sectional view and atop view of the terminal portion of the gate wiring. FIG. 4A1 is thecross-sectional view taken along line C1-C2 of FIG. 4A2.

In FIG. 4A1, a conductive layer 415 formed over a stack of the oxideinsulating layer 427 and the protective insulating layer 428 is aterminal electrode for connection which functions as an input terminal.Furthermore, in a terminal portion of FIG. 4A1, a first terminal 411formed using the same material as the gate wiring layer 421 c and aconnection electrode layer 412 formed using the same material as thesource wiring layer 422 overlap each other with the gate insulatinglayer 402 interposed therebetween, and are electrically connected toeach other through the transparent conductive layer 415. The conductivelayer 415 can be formed using the same light-transmitting material andthe same step as the pixel electrode layer 457.

FIGS. 4B1 and 4B2 respectively illustrate a cross-sectional view and atop view of a source wiring terminal portion. FIG. 4B1 is thecross-sectional view taken along line C3-C4 of FIG. 4B2.

In FIG. 4B1, a conductive layer 418 formed over a stack of the oxideinsulating layer 427 and the protective insulating layer 428 is theterminal electrode for connection which functions as an input terminal.Further in a terminal portion of FIG. 4B1, an electrode layer 416 formedusing the same material as the gate wiring layer 421 c is located belowand overlapped with a second terminal 414 electrically connected to thesource wiring with the gate insulating layer 402 interposedtherebetween. The electrode layer 416 is not electrically connected tothe second terminal 414, and a capacitor for preventing noise or staticelectricity can be formed if the potential of the electrode layer 416 isset to a potential different from that of the second terminal 414, suchas floating, GND, or 0 V. The second terminal 414 is electricallyconnected to the conductive layer 418. The conductive layer 418 can beformed using the same light-transmitting material and step as the pixelelectrode layer 457.

A plurality of gate wiring layers, source wiring layers, commonpotential lines, and power supply lines are provided depending on thepixel density. In the terminal portion, a plurality of first terminalsat the same potential as the gate wiring, a plurality of secondterminals at the same potential as the source wiring, a plurality ofthird terminals at the same potential as the power supply line, aplurality of fourth terminals at the same potential as the commonpotential line, and the like are arranged. There is no particularlimitation on the number of each of the terminals, and the number of theterminals may be determined by a practitioner as appropriate.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 5

In this embodiment, an example of a structure of a liquid-crystaldisplay device and a manufacturing method thereof will be described.

In this embodiment, a display device including a liquid crystal element(also referred to as a liquid crystal display element) is described;however, this embodiment is not limited thereto but can be applied to adisplay medium such as an electronic ink where contrast is changed byelectric operation.

Note that a display device in this specification includes a panel inwhich the display element is sealed, an integrated circuit (IC) foroperating the panel, and the like. In addition, an element substrateprovided with the display element includes, per pixel, means whichsupply current for the display element. Further, the “display device”includes the following modules in its category: a module including aconnector such as a flexible printed circuit (FPC), a tape automatedbonding (TAB) tape, or a tape carrier package (TCP) attached; a modulehaving a TAB tape or a TCP which is provided with a printed wiring boardat the end thereof; and a module having an integrated circuit (IC) whichis directly mounted on a display element by a chip on glass (COG)method.

The appearance and a cross section of a liquid crystal display panel,which is an embodiment of a display device, will be described withreference to FIGS. 5A1, 5A2, and 5B. FIGS. 5A1 and 5A2 are plan views ofpanels in which transistors 4010 and 4011 and a liquid crystal element4013 are sealed with a sealant 4005 between a first substrate 4001 and asecond substrate 4006. FIG. 5B is a cross-sectional view taken alongline M-N of FIGS. 5A1 and 5A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. Further, a signal linedriver circuit 4003 that is formed using a single crystal semiconductorfilm or a polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 5A1 illustrates anexample of mounting the signal line driver circuit 4003 by a COG method,and FIG. 5A2 illustrates an example of mounting the signal line drivercircuit 4003 by a TAB method.

A plurality of transistors are included in the pixel portion 4002 andthe scan line driver circuit 4004, which are formed over the firstsubstrate 4001. FIG. 5B illustrates the transistor 4010 included in thepixel portion 4002 and the transistor 4011 included in the scan linedriver circuit 4004. Insulating layers 4041, 4020, and 4021 are providedover the transistors 4010 and 4011.

Any of the high-reliable transistors including the oxide semiconductorlayer described in Embodiment 1 or 2 can be used as the transistors 4010and 4011. As the transistor 4011 for the driver circuit, the transistor450 described in Embodiment 1 or 2 can be used, and as the transistor4010 for a pixel, the transistor 460 described in Embodiment 1 or 2 canbe used. In this embodiment, the transistors 4010 and 4011 are n-channeltransistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region of an oxidesemiconductor layer in the transistor 4011 for the driver circuit. Theconductive layer 4040 is provided at the position overlapping with thechannel formation region of the oxide semiconductor layer, whereby theamount of shift in threshold voltage of the transistor 4011 can bereduced. A potential of the conductive layer 4040 may be the same as ordifferent from that of a gate electrode layer of the transistor 4011.The conductive layer 4040 can also function as a second gate electrode.Alternatively, the potential of the conductive layer 4040 may be GND or0 V, or the conductive layer 4040 may be in a floating state.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the transistor 4010. A counter electrode 4031of the liquid crystal element 4013 is formed on the second substrate4006. The liquid crystal element 4013 corresponds to a region where thepixel electrode 4030, the counter electrode 4031, and the liquid crystallayer 4008 overlap with each other. Note that the pixel electrode 4030and the counter electrode 4031 are respectively provided with aninsulating layer 4032 and an insulating layer 4033 each serving as analignment film.

Note that a light-transmitting substrate can be used as the firstsubstrate 4001 and the second substrate 4006; glass, ceramics, orplastics can be used. As plastic, a fiberglass-reinforced plastics (FRP)plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylicresin film can be used.

A columnar spacer 4035 which can be obtained in such a manner that aninsulating layer is selectively etched is provided to control a distance(a cell gap) between the pixel electrode 4030 and the counter electrode4031. Alternatively, a spherical spacer may be used.

The counter electrode 4031 is electrically connected to a commonpotential line provided over the same substrate as the transistor 4010.With the use of a common connection portion, the counter electrode 4031can be electrically connected to the common potential line throughconductive particles provided between the pair of substrates. Note thatthe conductive particles are included in the sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition including achiral agent at 5 wt % or more so as to improve the temperature range ispreferably used for the liquid crystal layer 4008. The liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral agent has a short response time of 1 msec or less and isoptically isotropic; therefore, alignment treatment is not necessary andviewing angle dependence is small.

In the transistor 4011, the insulating layer 4041 is formed in contactwith the oxide semiconductor layer. The insulating layer 4041 can beformed using a material and method similar to those of the oxideinsulating layer 427 described in Embodiment 1, and here a silicon oxidefilm formed by a sputtering method is used.

The protective insulating layer 4020 is formed over the insulating layer4041. The protective insulating layer 4020 can be formed using amaterial and a method which are similar to those of the protectiveinsulating layer 428 described in Embodiment 1. Here as the protectiveinsulating layer 4020, a silicon nitride film formed by a plasma CVDmethod is used.

The insulating layer 4021 is formed as the planarization insulatinglayer. As the insulating layer 4021, an organic material having heatresistance such as polyimide, an acrylic-based resin, abenzocyclobutene-based resin, polyamide, or an epoxy-based resin can beused. Other than such an organic material, it is also possible to use alow-dielectric constant material (low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating layers formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. A siloxane-based resin may include, as a substituent, anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

A formation method of the insulating layer 4021 is not particularlylimited, and the following method can be employed depending on thematerial: a sputtering method, an SOG method, a spin coating method, adipping method, a spray coating method, an inkjet method, screenprinting, offset printing, or the like. Further, the insulating layer4021 can be formed with a doctor knife, a roll coater, a curtain coater,a knife coater, or the like. The baking step of the insulating layer4021 also serves as annealing of the semiconductor layer, whereby thenumber of steps can be reduced.

The pixel electrode 4030 and the counter electrode 4031 can be formedusing a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

Alternatively, the pixel electrode 4030 and the counter electrode 4031can be formed using a conductive composition containing a conductivehigh molecule (also referred to as a conductive polymer). The pixelelectrode formed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In addition, a connection terminal electrode 4015 is formed from thesame conductive film as the pixel electrode 4030, and a terminalelectrode 4016 is formed from the same conductive film as source anddrain electrode layers of the transistor 4011.

The connection electrode 4015 is electrically connected to a terminalincluded in the FPC 4018 through an anisotropic conductive film 4019.

Note that FIGS. 5A1, 5A2, and 5B illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, the present invention is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

FIG. 6 illustrates an example of a liquid crystal display module whichis formed as a display device by using a transistor substrate 2600manufactured according to the manufacturing method disclosed in thisspecification.

The transistor substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including atransistor and the like, a display element 2604 including a liquidcrystal layer, and a coloring layer 2605 are provided between thesubstrates to form a display region.

The coloring layer 2605 is necessary to perform color display. In theRGB system, coloring layers corresponding to colors of red, green, andblue are provided for pixels. Polarizing plates 2606 and 2607 and adiffusion plate 2613 are provided outside the transistor substrate 2600and the counter substrate 2601.

A light source includes a cold-cathode tube 2610 and a reflector 2611. Acircuit board 2612 is connected to a wiring circuit portion 2608 of thetransistor substrate 2600 by a flexible wiring board 2609 and includesan external circuit such as a control circuit or a power source circuit.A retardation plate may be provided between the polarizing plate and theliquid crystal layer.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above process, a highly reliable liquid crystal displaypanel as a display device can be manufactured.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 6

In this embodiment, an example of operation of a driver circuit and apixel portion will be described, in which the driver circuit and thepixel portion are formed with transistors which are manufactured overone substrate.

In this embodiment, with use of a manufacturing method of a transistorin accordance with Embodiment 1, a pixel portion and a driver circuitportion are formed over one substrate. Note that the transistorsdescribed in Embodiment 1 are n-channel transistors, and the drivercircuit portion described here is limited to some circuits that can beconstituted by only n-channel TFTs.

FIG. 7A illustrates an example of a block diagram of an active-matrixdisplay device. The display device includes a pixel portion 5301, afirst scan line driver circuit 5302, a second scan line driver circuit5303, and a signal line driver circuit 5304 over a substrate 5300. Inthe pixel portion 5301, a plurality of signal lines extended from thesignal line driver circuit 5304 are placed and a plurality of scan linesextended from the first scan line driver circuit 5302 and the secondscan line driver circuit 5303 are placed. Note that pixels eachincluding a display element are arranged in matrix in regions where thescan lines and the signal lines intersect with each other. The substrate5300 of the display device is connected to a timing control circuit 5305(also referred to as a controller or a control IC) through a connectionportion such as an FPC (flexible printed circuit).

In FIG. 7A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over the same substrate 5300 as the pixel portion 5301.Consequently, the number of components of a driver circuit and the likethat are externally provided is reduced, so that costs can be reduced.In addition, the number of connection portions (such as FPC) for thesubstrate 5300 and the external driver circuit can be reduced; thus,reliability and yield can be improved.

The timing control circuit 5305 supplies, for example, a first scan linedriver circuit start signal (GSP1) and a scan line driver circuit clocksignal (GCLK1) to the first scan line driver circuit 5302. In addition,the timing control circuit 5305 supplies, for example, a second scanline driver circuit start signal (GSP2) (also referred to as a startpulse) and a scan line driver circuit clock signal (GCLK2) to the secondscan line driver circuit 5303.

The timing control circuit 5305 supplies a signal line driver circuitstart signal (SSP), a signal line driver circuit clock signal (SCLK),video signal data (DATA) (also simply referred to as a video signal), alatch signal (LAT) and the like to the signal line driver circuit 5304.Note that each clock signal may be a plurality of clock signals whosephases are shifted or may be supplied together with an inverted clocksignal (CKB) which is obtained by inverting the clock signal. One of thefirst scan line driver circuit 5302 and the second scan line drivercircuit 5303 can be omitted.

FIG. 7B illustrates a structure in which circuits with low drivingfrequency (e.g., the first scan line driver circuit 5302 and the secondscan line driver circuit 5303) are formed over the same substrate 5300as the pixel portion 5301 and the signal line driver circuit 5304 isformed over a substrate which is different from the pixel portion 5301.With use of the structure, even when transistors whose electrical fieldmobility is relatively lower are used, some of the driver circuits canbe formed over the same substrate as the pixel portion. Therefore,reduction in cost, improvement in yield, or the like can be achieved.

Next, an example of a structure of a signal line driver circuitincluding n-channel transistors and operation thereof are described withreference to FIGS. 8A and 8B.

The signal line driver circuit includes a shift register 5601 and aswitching circuit 5602. The switching circuit 5602 includes a pluralityof switching circuits 5602_1 to 5602_N (N is a natural number). Theswitching circuits 5602_1 to 5602_N each include a plurality oftransistors 5603_1 to 5603_k (k is a natural number). Here, thetransistors 5603_1 to 5603_k are n-channel transistors.

A connection relation in the signal line driver circuit is describedusing the switching circuit 5602_1 as an example. Respective firstterminals of the transistors 5603_1 to 5603_k are connected tocorresponding wirings 5604_1 to 5604_k. Second terminals of thetransistors 5603_1 to 5603_k are connected to signal lines S1 to Sk,respectively. Gates of the transistors 5603_1 to 5603_k are connected toa wiring 5605_1.

The shift register 5601 has a function of sequentially selecting theswitching circuits 5602_1 to 5602_N by sequentially outputting H-levelsignals (also referred to as an H signal or a signal at a high powersupply potential level) to wirings 5605_1 to 5605_N.

The switching circuit 5602_1 has a function of controlling conductionstates between the wirings 5604_1 to 5604_k and the signal lines S1 toSk (electrical continuity between the first terminals and the secondterminals), that is, a function of controlling whether potentials of thewirings 5604_1 to 5604_k are supplied to the signal lines S1 to Sk. Thatis, the switching circuit 5602_1 functions as a selector. Further, thetransistors 5603_1 to 5603_k have a function of controlling electricalcontinuity between the wirings 5604_1 to 5604_k and the signal lines S1to Sk, that is, a function of supplying potentials of the wirings 5604_1to 5604_k to the signal lines S1 to Sk. Thus, the transistors 5603_1 to5603_k each function as a switch.

Video signal data (DATA) is input to each of the wirings 5604_1 to5604_k. The video signal data (DATA) is an analog signal correspondingto image data or image signals in many cases.

Next, operation of the signal line driver circuit shown in FIG. 8A isdescribed with reference to a timing chart in FIG. 8B. FIG. 8Billustrates examples of signals Sout_1 to Sout_N and signals Vdata_1 toVdata_k. The signals Sout_1 to Sout_N are examples of output signalsfrom the shift register 5601. The signals Vdata_1 to Vdata_k areexamples of signals input to the wirings 5604_1 to 5604_k. Note that oneoperation period of the signal line driver circuit corresponds to onegate selection period in a display device. For example, one gateselection period is divided into periods T1 to TN. Each of the periodsT1 to TN is a period for writing the video signal data (DATA) into apixel in a selected row.

Note that signal waveform distortion and the like in drawings in thisembodiment are exaggerated for simplicity in some cases. Therefore, thepresent invention is not necessarily limited to such scales illustratedin the drawings.

In the periods T1 to TN, the shift register 5601 sequentially outputsH-level signals to the wirings 5605_1 to 5605_N. For example, in theperiod Ti, the shift register 5601 outputs a high-level signal to thewiring 5605_1. Then, the transistors 5603_1 to 5603_k are turned on, sothat the wirings 5604_1 to 5604_k and the signal lines S1 to Sk haveelectrical continuity. In this case, Data (S1) to Data (Sk) are input tothe wirings 5604_1 to 5604_k, respectively. The Data (S1) to Data (Sk)are input to pixels in the first to k-th columns in the selected rowthrough the transistors 5603_1 to 5603_k. In such a manner, in theperiods T1 to TN, the video signal data (DATA) are sequentially writteninto the pixels in the selected row by k columns.

Since the video signal data (DATA) is written to pixels by a pluralityof columns as described above, the number of video signal data (DATA) orthe number of wirings can be reduced. Consequently, the number ofconnections with an external circuit can be reduced. Moreover, the timefor writing can be extended when video signals are written into pixelsby a plurality of columns; thus, insufficient writing of video signalscan be prevented.

Note that a circuit including the transistor described in Embodiment 1or 2 can be used as the shift register 5601 and the switching circuit5602. In this case, all transistors included in the shift register 5601can be unipolar transistors.

Described next is constitution of a scan line driver circuit. The scanline driver circuit includes a shift register. Additionally, the scanline driver circuit may include a level shifter, a buffer, and the likein some cases. In the scan line driver circuit, when the clock signal(CLK) and the start pulse signal (SP) are input to the shift register, aselection signal is generated. The generated selection signal isbuffered and amplified by the buffer, and the resulting signal issupplied to a corresponding scan line. Gate electrodes of transistors inpixels of one line are connected to the scan line. Since the transistorsin the pixels of one line have to be turned on all at once, a bufferwhich can supply a large current is used.

One embodiment of a shift register which is used for part of the scanline driver circuit and/or the signal line driver circuit is describedwith reference to FIGS. 9A to 9D and FIGS. 10A and 10B.

The shift register includes first to Nth pulse output circuits 10_1 to10_N (N is a natural number greater than or equal to 3) (see FIG. 9A).In the shift register, a first clock signal CK1, a second clock signalCK2, a third clock signal CK3, and a fourth clock signal CK4 aresupplied from a first wiring 11, a second wiring 12, a third wiring 13,and a fourth wiring 14, respectively, to the first to Nth pulse outputcircuits 10_1 to 10_N.

A start pulse SP1 (a first start pulse) is input from a fifth wiring 15to the first pulse output circuit 10_1. To the nth pulse output circuit10_n of the second or subsequent stage (n is a natural number greaterthan or equal to 2 and less than or equal to N), a signal from the pulseoutput circuit of the preceding stage (such a signal is referred to as apreceding-stage signal OUT(n−1)) is input.

To the first pulse output circuit 10_1, a signal from the third pulseoutput circuit 10_3 of the stage following the next stage is input.Similarly, to the nth pulse output circuit 10_n of the second or itssubsequent stage, a signal from the (n+2)th pulse output circuit10_(n+2) of the stage following the next stage (such a signal isreferred to as a subsequent-stage signal OUT(n+2)) is input.

Therefore, the pulse output circuits of the respective stages outputfirst output signals OUT(1)(SR) to OUT(N)(SR) to be input to the pulseoutput circuit of the subsequent stage and/or the pulse output circuitof the stage before the preceding stage and second output signals OUT(1)to OUT(N) to be input to another circuit or the like. Note that sincethe subsequent-stage signal OUT(n+2) is not input to the last two stagesof the shift register as illustrated in FIG. 9A, a second start pulseSP2 and a third start pulse SP3 may be input to the stage before thelast stage and the last stage, respectively, for example.

Note that a clock signal (CK) is a signal whose level alternates betweenan H-level and an L-level (also referred to as an L signal or a signalat low power supply potential level) at regular intervals. Here, thefirst clock signal (CK1) to the fourth clock signal (CK4) are eachdelayed by ¼ cycle sequentially (i.e., they are 90° out of phase witheach other). In this embodiment, by using the first to fourth clocksignals (CK1) to (CK4), control or the like of driving of a pulse outputcircuit is performed. Although the clock signal is also represented byGCK or SCK depending on the driver circuit to which the signal is input,CK is used here.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in FIG. 9A, the first input terminal 21of the first pulse output circuit 10_1 is electrically connected to thefirst wiring 11, the second input terminal 22 of the first pulse outputcircuit 10_1 is electrically connected to the second wiring 12, and thethird input terminal 23 of the first pulse output circuit 10_1 iselectrically connected to the third wiring 13. In the second pulseoutput circuit 10_2, the first input terminal 21 is electricallyconnected to the second wiring 12, the second input terminal 22 iselectrically connected to the third wiring 13, and the third inputterminal 23 is electrically connected to the fourth wiring 14.

Each of the first to Nth pulse output circuits 10_1 to 10_N includes thefirst input terminal 21, the second input terminal 22, the third inputterminal 23, a fourth input terminal 24, a fifth input terminal 25, afirst output terminal 26, and a second output terminal 27 (see FIG. 9B).

In the first pulse output circuit 10_1, the first clock signal CK1 isinput to the first input terminal 21; the second clock signal CK2 isinput to the second input terminal 22; the third clock signal CK3 isinput to the third input terminal 23; the start pulse is input to thefourth input terminal 24; the subsequent stage signal OUT (3) is inputto the fifth input terminal 25; the first output signal OUT (1) (SR) isoutput from the first output terminal 26; and the second output signalOUT (1) is output from the second output terminal 27.

Note that in the first to Nth pulse output circuit 10_1 to 10_N, atransistor 28 with four terminals can be used besides the transistorwith three terminals (see FIG. 9C). Note that in this specification,when a transistor includes two gate electrodes with a semiconductorlayer therebetween, the gate electrode which is located below thesemiconductor layer is also referred to as a lower gate electrode andthe gate electrode which is located above the semiconductor layer isalso referred to as an upper gate electrode. The transistor 28 is anelement which can control electric current between an IN terminal and anOUT terminal with a first control signal G1 which is input to a lowergate electrode and a second control signal G2 which is input to an uppergate electrode.

When an oxide semiconductor is used for a semiconductor layer includinga channel formation region in a transistor, the threshold voltagesometimes shifts in a positive or negative direction depending on amanufacturing process. For that reason, the transistor in which an oxidesemiconductor is used for a semiconductor layer including a channelformation region preferably has a structure with which the thresholdvoltage can be controlled. The transistor 28 illustrated in FIG. 9C hasa structure in which gate electrodes are provided above and below achannel formation region with a gate insulating film interposed betweenthe upper gate electrode and the channel formation region and with agate insulating film interposed between the lower gate electrode and thechannel formation region, so that the threshold voltage of thetransistor 28 can be controlled to be a desired level by controlling apotential of the upper gate electrode and/or the lower gate electrode

Next, an example of a specific circuit configuration of the pulse outputcircuit is described with reference to FIG. 9D.

The pulse output circuit illustrated in FIG. 9D includes a first tothirteenth transistors 31 to 43. A signal or a power supply potential issupplied to the first transistor 31 to the thirteenth transistor 43 froma power supply line 51 to which a first high power supply potential VDDis supplied, a power supply line 52 to which a second high power supplypotential VCC is supplied, and a power supply line 53 to which a lowpower supply potential VSS is supplied, in addition to theabove-described first input terminal 21 to fifth input terminal 25, thefirst output terminal 26, and the second output terminal 27, which aredescribed above.

The relation of the power supply potentials of the power supply lines inFIG. 9D is as follows: the first power supply potential VDD is higherthan or equal to the second power supply potential VCC, and the secondpower supply potential VCC is higher than the third power supplypotential VSS. The first to fourth clock signals (CK1) to (CK4) aresignals which become H-level signals and L-level signals repeatedly at aregular interval. The potential is VDD when the clock signal is at theH-level, and the potential is VSS when the clock signal is at theL-level.

By making the potential VDD of the power supply line 51 higher than thepotential VCC of the power supply line 52, a potential applied to a gateelectrode of a transistor can be lowered, shift in threshold voltage ofthe transistor can be reduced, and deterioration of the transistor canbe suppressed without an adverse effect on the operation of thetransistor.

Note that as in FIG. 9D, the transistor 28 with four terminals which isillustrated in FIG. 9C is preferably used as the first transistor 31 andthe sixth to ninth transistors 36 to 39 among the first to thirteenthtransistors 31 to 43.

The first transistor 31 and the sixth to ninth transistors 36 to 39 needto switch a potential of a node to which one electrode serving as asource or a drain is connected depending on a control signal of the gateelectrode, and are desired to enable a malfunction of the pulse outputcircuit to be reduced by quick response (sharp rising of on current) tothe control signal input to the gate electrode. Thus, by using thetransistor 28 with four terminals, the threshold voltage can becontrolled, and a malfunction of the pulse output circuit can be furtherreduced. Note that although the first control signal G1 and the secondcontrol signal G2 are the same control signals in FIG. 9D, the firstcontrol signal G1 and the second control signal G2 may be differentcontrol signals.

In FIG. 9D, a first terminal of the first transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the firsttransistor 31 is electrically connected to a first terminal of the ninthtransistor 39, and gate electrodes (a lower gate electrode and an uppergate electrode) of the first transistor 31 are electrically connected tothe fourth input terminal 24.

A first terminal of the second transistor 32 is electrically connectedto the power supply line 53, a second terminal of the second transistor32 is electrically connected to the first terminal of the ninthtransistor 39, and a gate electrode of the second transistor 32 iselectrically connected to a gate electrode of the fourth transistor 34.

A first terminal of the third transistor 33 is electrically connected tothe first input terminal 21, and a second terminal of the thirdtransistor 33 is electrically connected to the first output terminal 26.

A first terminal of the fourth transistor 34 is electrically connectedto the power supply line 53, and a second terminal of the fourthtransistor 34 is electrically connected to the first output terminal 26.

A first terminal of the fifth transistor 35 is electrically connected tothe power supply line 53, a second terminal of the fifth transistor 35is electrically connected to the gate electrode of the second transistor32 and the gate electrode of the fourth transistor 34, and a gateelectrode of the fifth transistor 35 is electrically connected to thefourth input terminal 24.

A first terminal of the sixth transistor 36 is electrically connected tothe power supply line 52, a second terminal of the sixth transistor 36is electrically connected to the gate electrode of the second transistor32 and the gate electrode of the fourth transistor 34, and gateelectrodes (a lower gate electrode and an upper gate electrode) of thesixth transistor 36 are electrically connected to the fifth inputterminal 25.

A first terminal of the seventh transistor 37 is electrically connectedto the power supply line 52, a second terminal of the seventh transistor37 is electrically connected to a second terminal of the eighthtransistor 38, and gate electrodes (a lower gate electrode and an uppergate electrode) of the seventh transistor 37 are electrically connectedto the third input terminal 23.

A first terminal of the eighth transistor 38 is electrically connectedto the gate electrode of the second transistor 32 and the gate electrodeof the fourth transistor 34, and gate electrodes (a lower gate electrodeand an upper gate electrode) of the eighth transistor 38 is electricallyconnected to the second input terminal 22.

The first terminal of the ninth transistor 39 is electrically connectedto the second terminal of the first transistor 31 and the secondterminal of the second transistor 32, a second terminal of the ninthtransistor 39 is electrically connected to a gate electrode of the thirdtransistor 33 and a gate electrode of the tenth transistor 40, and gateelectrodes (a lower gate electrode and an upper gate electrode) of theninth transistor 39 are electrically connected to the power supply line52.

A first terminal of the tenth transistor 40 is electrically connected tothe first input terminal 21, a second terminal of the tenth transistor40 is electrically connected to the second output terminal 27, and thegate electrode of the tenth transistor 40 is electrically connected tothe second terminal of the ninth transistor 39.

A first terminal of the eleventh transistor 41 is electrically connectedto the power supply line 53, a second terminal of the eleventhtransistor 41 is electrically connected to the second output terminal27, and a gate electrode of the eleventh transistor 41 is electricallyconnected to the gate electrode of the second transistor 32 and the gateelectrode of the fourth transistor 34.

A first terminal of the twelfth transistor 42 is electrically connectedto the power supply line 53, a second terminal of the twelfth transistor42 is electrically connected to the second output terminal 27, and agate electrode of the twelfth transistor 42 is electrically connected tothe gate electrodes (the lower gate electrode and the upper gateelectrode) of the seventh transistor 37.

A first terminal of the thirteenth transistor 43 is electricallyconnected to the power supply line 53, a second terminal of thethirteenth transistor 43 is electrically connected to the first outputterminal 26, and a gate electrode of the thirteenth transistor 43 iselectrically connected to the gate electrodes (the lower gate electrodeand the upper gate electrode) of the seventh transistor 37.

In FIG. 9D, a portion where the gate electrode of the third transistor33, the gate electrode of the tenth transistor 40, and the secondterminal of the ninth transistor 39 are connected is referred to as anode A. Further, the portion where the gate electrode of the secondtransistor 32, the gate electrode of the fourth transistor 34, thesecond terminal of the fifth transistor 35, the second terminal of thesixth transistor 36, the first terminal of the eighth transistor 38, andthe gate electrode of the eleventh transistor 41 are connected isreferred to as a node B (see FIG. 10A).

In FIG. 10A, the signals that are input or output to/from the first tofifth input terminals 21 to 25 and the first and second output terminals26 and 27 in the case where the pulse output circuit illustrated in FIG.9D is applied to the first pulse output circuit 10_1 are shown.

Specifically, the first clock signal CK1 is input to the first inputterminal 21; the second clock signal CK2 is input to the second inputterminal 22; the third clock signal CK3 is input to the third inputterminal 23; the start pulse is input to the fourth input terminal 24;the subsequent-stage signal OUT(3) is input to the fifth input terminal25; the first output signal OUT(1)(SR) is output from the first outputterminal 26; and the second output signal OUT(1) is output from thesecond output terminal 27.

Note that a transistor is an element with at least three terminals of agate, a drain, and a source. The transistor has a channel region betweena drain region and a source region, and current can flow through thedrain region, the channel region, and the source region. Here, since thesource and the drain of the transistor may change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Therefore, a regionfunctioning as source and drain is not called the source or the drain insome cases. In such a case, one of the source and the drain may bereferred to as a first terminal and the other thereof may be referred toas a second terminal, for example.

Note that in FIG. 10A, a capacitor may be provided independently inorder to perform bootstrap operation by bringing the node A into afloating state. Furthermore, a capacitor having one electrodeelectrically connected to the node B may be additionally provided inorder to hold a potential of the node B.

FIG. 10B shows a timing chart of the shift register including aplurality of pulse output circuits shown in FIG. 10A. Note that when theshift register is included in a scan line driver circuit, a period 61 inFIG. 10B corresponds to a vertical retrace period and a period 62corresponds to a gate selection period.

Note that by providing the ninth transistor 39 in which the second powersupply potential VCC is applied to the gate as illustrated in FIG. 10A,the following advantages before and after bootstrap operation areprovided.

Without the ninth transistor 39 in which the second power supplypotential VCC is applied to the gate electrode, if a potential of thenode A is raised by bootstrap operation, a potential of the source whichis the second terminal of the first transistor 31 rises to a valuehigher than the first power supply potential VDD. Then, the source ofthe first transistor 31 is switched to the first terminal side, that is,on the power supply line 51 side. Consequently, in the first transistor31, a high bias voltage is applied and thus significant stress isapplied between the gate and the source and between the gate and thedrain, which might cause deterioration of the transistor.

Therefore, with the ninth transistor 39 in which the second power supplypotential VCC is applied to the gate electrode, an increase in potentialof the second terminal of the first transistor 31 can be prevented whilethe potential of the node A is raised by bootstrap operation. In otherwords, the placement of the ninth transistor 39 can lower the value of anegative bias voltage applied between the gate and the source of thefirst transistor 31. Thus, the circuit configuration in this embodimentcan reduce a negative bias voltage applied between the gate and thesource of the first transistor 31, so that deterioration of the firsttransistor 31 due to stress can be suppressed.

Note that the ninth transistor 39 can be provided anywhere as long asthe first terminal and the second terminal of the ninth transistor 39are connected to the second terminal of the first transistor 31 and thegate of the third transistor 33 respectively. In the case of employing ashift register including a plurality of pulse output circuits of thisembodiment, the ninth transistor 39 may be omitted in a signal linedriver circuit in which the number of stages is larger than that of ascan line driver circuit, in order to reduce the number of transistors.

When an oxide semiconductor is used for each of the semiconductor layersof the first to thirteenth transistors 31 to 43, the amount of offcurrent of the transistors can be reduced, the amount of on current andfield-effect mobility can be increased, and the rate of deteriorationcan be decreased, whereby malfunctions of the circuit can be reduced.Moreover, the degree of deterioration of the transistor using an oxidesemiconductor by application of a high potential to a gate electrode issmaller than that of a transistor using amorphous silicon. Therefore,even when the first power supply potential VDD is supplied to the powersupply line to which the second power supply potential VCC is supplied,similar operation can be performed and the number of power supply linesprovided between the circuits can be reduced, whereby size reduction ina circuit can be achieved.

Note that a similar function is obtained even when the connectionrelation is changed so that a clock signal that is supplied to the gateelectrodes (the lower gate electrode and the upper gate electrode) ofthe seventh transistor 37 from the third input terminal 23 and a clocksignal that is supplied to the gate electrodes (the lower gate electrodeand the upper gate electrode) of the eighth transistor 38 from thesecond input terminal 22 are supplied from the second input terminal 22and the third input terminal 23, respectively.

In the shift register shown in FIG. 10A, a state of the seventhtransistor 37 and the eighth transistor 38 is changed so that both theseventh transistor 37 and the eighth transistor 38 are on, then theseventh transistor 37 is off and the eighth transistor 38 is on, andthen the seventh transistor 37 and the eighth transistor 38 are off;thus, the fall in potential of the node B, which is caused by fall inpotentials of the second input terminal 22 (CK2) and the third inputterminal 23 (CK3), is caused twice by fall in potential of the gateelectrode of the seventh transistor 37 and fall in potential of the gateelectrode of the eighth transistor 38.

On the other hand, in the case where a state of the seventh transistor37 and the eighth transistor 38 in the shift register illustrated inFIG. 10A is changed so that both the seventh transistor 37 and theeighth transistor 38 are on, then the seventh transistor 37 is on andthe eighth transistor 38 is off, and then the seventh transistor 37 andthe eighth transistor 38 are off; the fall in potential of the node B,which is caused by fall in potentials of the second input terminal 22(CK2) and the third input terminal 23 (CK3), is caused only once by fallin potential of the gate electrode of the eighth transistor 38.

Therefore, the connection relation, that is, the clock signal CK3 issupplied from the third input terminal 23 to the gate electrodes (thelower electrode and the upper electrode) of the seventh transistor 37and the clock signal CK2 is supplied from the second input terminal 22to the gate electrodes (the lower gate electrode and the upper gateelectrode) of the eighth transistor 38, is preferable. This is becausefluctuation of the potential of the node B can be reduced and noise canbe reduced.

In such a manner, an H-level signal is regularly supplied to the node Bin a period during which the potentials of the first output terminal 26and the second output terminal 27 are held at L level; thus, amalfunction of the pulse output circuit can be suppressed.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 7

In this embodiment, an example of a liquid crystal display device willbe described as one embodiment of a display device with reference toFIGS. 11 to 24, which includes the transistor described in Embodiment 1or 2 and a liquid crystal element as a display element.

First, a vertical alignment (VA) liquid crystal display device is shown.The VA liquid crystal display device has a kind of form in whichalignment of liquid crystal molecules of a liquid crystal display panelis controlled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied. In this embodiment, in particular, apixel is divided into some regions (for example, two to four subpixels),and molecules are aligned in different directions in their respectiveregions. This is referred to as multi-domain or multi-domain design.Hereinafter, a liquid crystal display device of multi-domain design isdescribed.

FIGS. 12 and 13 illustrate a pixel electrode and a counter electrode,respectively. Note that FIG. 12 is a plan view illustrating a substrateside where the pixel electrode is formed. FIG. 11 illustrates across-sectional structure taken along a section line E-F in FIG. 12.FIG. 13 is a plan view of a substrate side where the counter electrodeis formed. Hereinafter, description is made with reference to thesedrawings.

In FIG. 11, a substrate 600 provided with a transistor 628, a pixelelectrode layer 624 connected to the transistor 628, and a storagecapacitor portion 630 overlaps with a counter substrate 601 providedwith a counter electrode layer 640 and the like, and liquid crystals areinjected between both the substrates.

The counter substrate 601 is provided with a coloring film 636 and thecounter electrode layer 640, and protrusions 644 are formed on thecounter electrode layer 640. An alignment film 648 is formed over thepixel electrode layer 624, and an alignment film 646 is similarly formedon the counter electrode layer 640 and the protrusions 644. A liquidcrystal layer 650 is formed between the substrate 600 and the countersubstrate 601.

The transistor 628, the pixel electrode layer 624 connected thereto, andthe storage capacitor portion 630 are formed over the substrate 600. Thepixel electrode layer 624 is connected to a wiring 618 through a contacthole 623 formed in an insulating film 620, an insulating film 621, andan insulating film 622. The transistor described in Embodiment 1 or 2can be used as the transistor 628 as appropriate. Further, the storagecapacitor portion 630 includes a first capacitor wiring 604 which isformed concurrently with a gate wiring 602 of the transistor 628; a gateinsulating layer 606; and a second capacitor wiring 617 which is formedconcurrently with a wiring 616 and the wiring 618.

The pixel electrode layer 624, the liquid crystal layer 650, and thecounter electrode layer 640 overlap with each other, whereby a liquidcrystal element is formed.

FIG. 12 illustrates a planar structure over the substrate 600. The pixelelectrode layer 624 is formed using the material described inEmbodiment 1. The pixel electrode layer 624 is provided with slits 625.The slits 625 are formed to control alignment of the liquid crystals.

A transistor 629, a pixel electrode layer 626 connected thereto, and astorage capacitor portion 631 which are illustrated in FIG. 12 can beformed similarly to the transistor 628, the pixel electrode layer 624,and the storage capacitor portion 630, respectively. Both thetransistors 628 and 629 are connected to the wiring 616. One pixel ofthis liquid crystal display panel includes the pixel electrode layers624 and 626. That is, the pixel electrode layers 624 and 626 constitutesubpixels. Although the pixel includes two subpixels in this embodiment,the pixel can include more than two subpixels.

FIG. 13 illustrates a planar structure of the counter substrate side.The counter electrode layer 640 is preferably formed using a materialsimilar to that of the pixel electrode layer 624. The protrusions 644which control the alignment of the liquid crystals are formed on thecounter electrode layer 640. Note that in FIG. 13, the pixel electrodelayers 624 and 626 formed over the substrate 600 are represented bydashed lines, and the counter electrode layer 640 and the pixelelectrode layers 624 and 626 overlap with each other.

FIG. 14 shows an equivalent circuit of this pixel structure. Both thetransistors 628 and 629 are connected to the gate wiring 602 and thewiring 616. In this case, when potentials of the capacitor wiring 604and a capacitor wiring 605 are different from each other, operations ofliquid crystal elements 651 and 652 can vary. That is, alignment of theliquid crystal is precisely controlled and a viewing angle is increasedby individual control of potentials of the capacitor wirings 604 and605.

When a voltage is applied to the pixel electrode layer 624 provided withthe slits 625, a distorted electric field (an oblique electric field) isgenerated in the vicinity of the slits 625. The slits 625 and theprotrusions 644 on the counter substrate 601 side are disposed so as notto overlap with each other, thereby effectively generating the obliqueelectric field to control alignment of the liquid crystals, and thus thedirection in which liquid crystals are aligned is different depending onthe location. That is, a viewing angle of the liquid crystal displaypanel is increased by domain multiplication.

Next, another VA liquid crystal display device, which is different fromthe above-described device, is described with reference to FIG. 15, FIG.16, FIG. 17, and FIG. 18.

FIG. 15 and FIG. 16 each show a pixel structure of a VA liquid crystaldisplay panel. FIG. 16 is a plane view of the substrate 600. FIG. 15shows a cross-sectional structure taken along a section line Y-Z in FIG.16.

In this pixel structure, a plurality of pixel electrodes are included inone pixel, and a transistor is connected to each of the pixelelectrodes. The transistors are driven by different gate signals. Thatis, a multi-domain pixel has a structure in which signals supplied tothe respective pixel electrodes are individually controlled.

The pixel electrode layer 624 is connected to the transistor 628 via thewiring 618 in the contact hole 623. Further, in a contact hole 627, thepixel electrode layer 626 is connected to the transistor 629 via awiring 619.

As each of the transistors 628 and 629, the transistor described inEmbodiment 1 or 2 can be used as appropriate. The gate wiring 602 of thetransistor 628 and a gate wiring 603 of the transistor 629 are separatedso that different gate signals can be given thereto. In contrast, thewiring 616 functioning as a data line is used in common for thetransistors 628 and 629. In addition, a capacitor wiring 690 is providedbelow the wiring 618 and a wiring 619 with the gate insulating layer 606therebetween.

The shape of the pixel electrode layer 624 is different from that of thepixel electrode layer 626, and the pixel electrode layers are separatedby slits 625. The pixel electrode layer 626 is formed so as to surroundthe external side of the pixel electrode layer 624 which spreads into aV shape. Voltage application is made to vary between the pixel electrodelayers 624 and 626 by the transistors 628 and 629, so that alignment ofthe liquid crystals is controlled. FIG. 18 shows an equivalent circuitof this pixel structure. The transistor 628 is connected to the gatewiring 602, and the transistor 629 is connected to the gate wiring 603.Both the transistors 628 and 629 are connected to the wiring 616. Whendifferent gate signals are supplied to the gate wirings 602 and 603,operations of liquid crystal elements 651 and 652 can vary. In otherwords, when operation of the transistors 628 and 629 is individuallycontrolled, alignment of liquid crystal in the liquid crystal elements651 and 652 can be precisely controlled; accordingly, viewing angle canbe increased.

The counter substrate 601 is provided with the coloring film 636 and thecounter electrode layer 640. A planarization film 637 is formed betweenthe coloring film 636 and the counter electrode layer 640 to preventalignment disorder of the liquid crystals. FIG. 17 illustrates a planarstructure of the counter substrate side. The counter electrode layer 640is an electrode shared by different pixels and slits 641 are formed. Theslits 641 and the slits 625 on the pixel electrode layer 624 and 626sides are disposed so as not to overlap with each other so that anoblique electric field is effectively generated, whereby the alignmentof the liquid crystals can be controlled. Accordingly, the alignment ofthe liquid crystals can be varied in different places, so that theviewing angle is widened. Note that in FIG. 17, the pixel electrodelayers 624 and 626 formed over the substrate 600 are indicated by dashedlines, and the counter electrode layer 640 and the pixel electrodelayers 624 and 626 overlap with each other.

The alignment film 648 is formed over the pixel electrode layer 624 andthe pixel electrode layer 626, and similarly, the alignment film 646 isprovided on the counter electrode layer 640. A liquid crystal layer 650is formed between the substrate 600 and the counter substrate 601. Thepixel electrode layer 624, the liquid crystal layer 650, and the counterelectrode layer 640 overlap with each other to form a first liquidcrystal element 651. The pixel electrode layer 626, the liquid crystallayer 650, and the counter electrode layer 640 overlap with each otherto form a second liquid crystal element 652. The pixel structure of thedisplay panel illustrated in FIG. 15 to FIG. 18 is a multi-domainstructure in which the first liquid crystal element and the secondliquid crystal element are provided in one pixel.

Next, a liquid crystal display device in a horizontal electric fieldmode is shown. In a horizontal electric field mode, an electric field isapplied in a horizontal direction with respect to liquid crystalmolecules in a cell, whereby liquid crystals are driven to express grayscales. In accordance with this method, a viewing angle can be expandedto about 180°. Hereinafter, a liquid crystal display device in thehorizontal electric field mode is described.

In FIG. 19, the substrate 600 provided with an electrode layer 607, thetransistor 628, and the pixel electrode layer 624 overlaps with thecounter substrate 601, and liquid crystals are injected therebetween.The counter substrate 601 is provided with the coloring film 636, theplanarization film 637, and the like. The pixel electrode is providedfor the substrate 600, and not for the counter substrate 601. Inaddition, the liquid crystal layer 650 is formed between the substrate600 and the counter substrate 601, and the alignment film 646 and thealignment film 648 are provided between the liquid crystal layer 650 andthe counter substrate 601 and between the liquid crystal layer 650 andthe substrate 600.

The electrode layer 607 and the capacitor wiring 604 connected to theelectrode layer 607, and the transistor 628 are formed over thesubstrate 600. The capacitor wiring 604 can be formed concurrently withthe gate wiring 602 of the transistor 628. The transistor described inany of Embodiments 1 to 5 can be used as the transistor 628. Theelectrode layer 607 can be formed using a material similar to that ofthe pixel electrode layer described in Embodiments 1 or 2. The electrodelayer 607 is divided almost in a pixel form. Note that the gateinsulating layer 606 is formed over the electrode layer 607 and thecapacitor wiring 604.

The wirings 616 and 618 of the transistor 628 are formed over the gateinsulating layer 606. The wiring 616 is a data line through which avideo signal travels, extends in one direction in a liquid crystaldisplay panel, is connected to a source or drain region of thetransistor 628, and functions as one of source and drain electrodes. Thewiring 618 functions as the other of the source and drain electrodes andis connected to the pixel electrode layer 624.

The insulating film 620 and the insulating film 621 are formed over thewirings 616 and 618. Over the insulating film 621, the pixel electrodelayer 624 is formed to be connected to the wiring 618 through a contacthole 623 formed in the insulating film 620 and the insulating film 621.The pixel electrode layer 624 can be formed using a material similar tothat of the pixel electrode layer 457 described in Embodiment 3.

Thus, the transistor 628 and the pixel electrode layer 624 connectedthereto are formed over the substrate 600. Note that a storage capacitoris formed with the electrode layer 607 and the pixel electrode layer624.

FIG. 20 is a plan view illustrating a structure of the pixel electrode.FIG. 19 illustrates a cross-sectional structure taken along a sectionline O-P in FIG. 20. The pixel electrode layer 624 is provided withslits 625. The slits 625 are provided for controlling alignment ofliquid crystals.

In that case, an electric field is generated between the electrode layer607 and the pixel electrode layer 624. The thickness of the gateinsulating layer 606 formed between the electrode layer 607 and thepixel electrode layer 624 is 50 nm to 200 nm, which is much smaller thanthe thickness of the liquid crystal layer of 2 μm to 10 μm. Thus, anelectric field is generated substantially in parallel (in a horizontaldirection) to the substrate 600. By this electric field, alignment ofthe liquid crystal is controlled, and liquid crystal molecules arehorizontally rotated with use of the electric field in the directionalmost parallel to the substrate. In this case, since the liquid crystalmolecules are parallel to the substrate in any state, contrast is lessaffected by change in angle of viewing, and a viewing angle isincreased. In addition, since both the electrode layer 607 and the pixelelectrode layer 624 are light-transmitting electrodes, an aperture ratiocan be improved.

Next, a different example of a liquid crystal display device in ahorizontal electric field mode is shown.

FIGS. 21 and 22 each illustrate a pixel structure of an IPS liquidcrystal display device. FIG. 22 is a plan view. FIG. 21 illustrates across-sectional structure taken along a section line V-W in FIG. 22.

In FIG. 21, the substrate 600 provided with the transistor 628 and thepixel electrode layer 624 connected thereto overlaps with the countersubstrate 601, and liquid crystals are injected therebetween. Thecounter substrate 601 is provided with the coloring film 636, theplanarization film 637, and the like. Note that a counter electrode isnot provided on the counter substrate 601 side. In addition, the liquidcrystal layer 650 is formed between the substrate 600 and the countersubstrate 601, and the alignment film 646 and the alignment film 648 areprovided between the liquid crystal layer 650 and the counter substrate601 and between the liquid crystal layer 650 and the substrate 600.

A common potential line 609 and the transistor 628 are formed over thesubstrate 600. The common potential line 609 can be formed concurrentlywith the gate wiring 602 of the transistor 628. Note that the transistordescribed in Embodiment 1 or 2 can be used for the transistor 628.

The wirings 616 and 618 of the transistor 628 are formed over the gateinsulating layer 606. The wiring 616 is a data line which supplies datasignals in the liquid crystal display panel, connected to a sourceregion or a drain region of the transistor 628, and functions as one ofsource and drain electrodes. The wiring 618 is a wiring connected to thepixel electrode layer 624 and functions as the other electrode of thetransistor 628.

The insulating films 620 and 621 are formed over the wirings 616 and618. Over the insulating film 620 and the insulating film 621, the pixelelectrode layer 624 which is connected to the wiring 618 through thecontact hole 623 formed in the insulating film 620 and the insulatingfilm 621 is formed. The pixel electrode layer 624 can be formed using amaterial similar to that of the pixel electrode layer 457 described inEmbodiment 3. Note that, as illustrated in FIG. 22, the pixel electrodelayer 624 is formed such that a horizontal electric field is formedbetween the pixel electrode layer 624 and a comb-like electrode that isformed at the same time as the common potential line 609. Further, acomb-like portion of the pixel electrode layer 624 and the comb-likeelectrode that is formed at the same time as the common potential line609 are disposed so as not to overlap with each other.

The alignment of the liquid crystals is controlled by an electric fieldgenerated between a potential applied to the pixel electrode layer 624and a potential of the common potential line 609. The liquid crystalmolecules are horizontally rotated using the electric field which isapproximately parallel to the substrate. In this case, since the liquidcrystal molecules are parallel to the substrate in any state, contrastor the like is less affected by change in angle of viewing, and aviewing angle is increased.

Thus, the transistor 628 and the pixel electrode layer 624 connectedthereto are formed over the substrate 600. The storage capacitor isformed with the common potential line 609, the gate insulating layer606, and a capacitor electrode 615. The capacitor electrode 615 and thepixel electrode layer 624 are connected through a contact hole 633.

Next, a mode of a liquid crystal display device in a TN mode isdescribed.

FIG. 23 and FIG. 24 each illustrate a pixel structure of a TN liquidcrystal display device. FIG. 24 is a plane view. FIG. 23 illustrates across-sectional structure taken along a section line K-L in FIG. 24.

The pixel electrode layer 624 is connected to the transistor 628 via thewiring 618 in the contact hole 623. The wiring 616 functioning as a dataline is connected to the transistor 628. The transistor described inEmbodiment 1 or 2 can be used for the transistor 628.

The pixel electrode layer 624 can be formed using a material similar tothat of the pixel electrode layer 457 described in Embodiment 3.

The counter substrate 601 is provided with the coloring film 636 and thecounter electrode layer 640. The planarization film 637 is formedbetween the coloring film 636 and the counter electrode layer 640 toprevent alignment disorder of the liquid crystals. The liquid crystallayer 650 is formed between the pixel electrode layer 624 and thecounter electrode layer 640 with the alignment films 646 and 648therebetween. The pixel electrode layer 624, the liquid crystal layer650, and the counter electrode layer 640 overlap with each other to forma liquid crystal element.

The coloring film 636 may be formed on the substrate 600 side. Apolarizing plate is attached to a surface of the substrate 600, which isthe reverse of the surface provided with the transistor, and apolarizing plate is attached to a surface of the counter substrate 601,which is the reverse of the surface provided with the counter electrodelayer 640.

Through the above steps, a liquid crystal display device with highaperture ratio can be manufactured.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 8

A display device disclosed in this specification can be applied to avariety of electronic appliances (including game machines). Examples ofelectronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 25A illustrates an example of a mobile phone. A mobile phone 1100includes a housing 1101, a display portion 1102 incorporated in thehousing 1101, an operation button 1103, an external connection port1104, a speaker 1105, a microphone 1106, and the like.

Information can be inputted to the mobile phone 1100 in FIG. 25A whenthe display portion 1102 is touched with a finger or the like. Operationsuch as telephone call and sending and receiving mails can be conductedwhen the display portion 1102 is touched with a finger or the like.

There are mainly three screen modes of the display portion 1102. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or texting, a text input modemainly for inputting text is selected for the display portion 1102 sothat characters displayed on a screen can be input. In that case, it ispreferable to display a keyboard or number buttons on the displayportion 1102 with high recognition.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1100, display on the screen of the display portion 1102 canbe automatically switched by determining the direction of the mobilephone 1100 (whether the mobile phone 1100 is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are changed by touching the display portion 1102 orusing the operation button 1103 of the housing 1101. Alternatively, thescreen modes can be switched depending on kinds of images displayed onthe display portion 1102. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Further, in the input mode, a signal is detected by an optical sensor inthe display portion 1102 and if input by touching the display portion1102 is not performed for a certain period, the screen mode may becontrolled so as to be switched from the input mode to the display mode.

The display portion 1102 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken bytouching the display portion 1102 with the palm or the finger, wherebypersonal authentication can be performed. Further, by using a lightsource which emits a near-infrared light, an image of a finger vein, apalm vein, or the like can be taken. Here, the display portion 1102includes a plurality of transistors 460 described in Embodiment 1 or 2.Since the transistor 460 has a light-transmitting property, a lightsensor can be provided below the transistor 460. Further, in the case ofusing a light source which emits a near-infrared light, light is notblocked by the transistor 460, so that an object can be irradiated witha near-infrared light with high light intensity.

FIG. 25B also illustrates an example of a mobile phone. A portableinformation terminal whose example is illustrated in FIG. 25B can have aplurality of functions. For example, such a portable informationterminal incorporates a computer and can have a function of processing avariety of pieces of data, in addition to a telephone function.

The portable information terminal illustrated in FIG. 25B has a housing1800 and a housing 1801. The housing 1800 includes a display panel 1802,a speaker 1803, a microphone 1804, a pointing device 1806, a camera lens1807, an external connection terminal 1808, and the like. The housing1801 includes a keyboard 1810, an external memory slot 1811, and thelike. In addition, an antenna is incorporated in the housing 1801.

The display panel 1802 is provided with a touch panel. A plurality ofoperation keys 1805 which are displayed as images are illustrated bydashed lines in FIG. 25B.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

The display device of the present invention can be used for the displaypanel 1802 and the direction of display is changed appropriatelydepending on an application mode. Further, the display device isprovided with the camera lens 1807 on the same surface as the displaypanel 1802, and thus it can be used as a video phone. The speaker 1803and the microphone 1804 can be used for recording, and playing sound,etc. as well as voice calls. Moreover, the housings 1800 and 1801 in astate where they are developed as illustrated in FIG. 25B can shift sothat one is lapped over the other by sliding; therefore, the size of theportable information terminal can be reduced, which makes the portableinformation terminal suitable for being carried.

The external connection terminal 1808 is an input-output terminal forinputting power supply and information communication, and storingelectric power and data communication with a personal computer or thelike are possible. Moreover, a storage medium can be inserted into theexternal memory slot 1811 so that a large amount of data can be storedand can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 26A illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. Switching channelsand volume can be controlled by an operation key 9609 of the remotecontroller 9610 so that an image displayed on the display portion 9603can be controlled. Furthermore, the remote controller 9610 may beprovided with a display portion 9607 for displaying data output from theremote controller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 26B illustrates an example of a digital photo frame. For example,in the digital photo frame 9700, a display portion 9703 is incorporatedin a housing 9701. The display portion 9703 can display a variety ofimages. For example, the display portion 9703 can display image datataken with a digital camera or the like and function as a normal photoframe

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (such as a USB terminal), anexternal memory slot, and the like. Although these components may beprovided on the surface on which the display portion is provided, it ispreferable to provide them on the side surface or the back surface forthe design of the digital photo frame 9700. For example, a memorystoring image data taken with a digital camera is inserted in theexternal memory slot of the digital photo frame, whereby the image datacan be transferred and then displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly to be displayed.

FIG. 27 is a portable game machine and includes two housings, a housing9881 and a housing 9891, which are connected with a joint portion 9893so that the portable game machine can be opened or folded. A displayportion 9882 and a display portion 9883 are incorporated in the housing9881 and the housing 9891, respectively.

In addition, the portable game machine illustrated in FIG. 27 includes aspeaker portion 9884, an external memory slot 9886, an LED lamp 9890, aninput means (an operation key 9885, a connection terminal 9887, a sensor9888 (a sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays), or a microphone 9889), and the like. It is needless to say thatthe structure of the portable game machine is not limited to the aboveand other structures provided with at least a display device disclosedin this specification may be employed. The portable game machine mayinclude an additional accessory as appropriate. The portable gamemachine illustrated in FIG. 27 has a function of reading a program ordata stored in a recording medium to display it on the display portion,and a function of sharing information with another portable game machineby wireless communication. The portable game machine illustrated in FIG.27 can have various functions without limitation to the above.

As described above, the display devices described in other embodimentscan be provided in display portions of a variety of electronicappliances as described above.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 9

In this embodiment, an example of a structure of a storage capacitorwhich is different from that in Embodiment 3 will be described withreference to FIGS. 28A and 28B. FIGS. 28A and 28B are each across-sectional view of the transistor 460 and a storage capacitor inthe pixel portion. Note that FIGS. 28A and 28B are the same as FIG. 3except for a structure of the storage capacitor; therefore, the sameportions are denoted by the same reference numerals and detaileddescription of the same portions is omitted.

FIG. 28A illustrates an example in which a storage capacitor is formedwith the pixel electrode layer 457 and a capacitor wiring layer 432 inwhich the oxide insulating layers 426 and 427, the protective insulatinglayer 428, and the planarization insulating layer 456 are used asdielectrics. Since the capacitor wiring layer 432 is formed using thesame light-transmitting material in the same step as the sourceelectrode layer of the transistor 460 in the pixel portion, thecapacitor wiring layer 432 is arranged so as not to overlap with asource wiring layer of the transistor 460.

In the storage capacitor illustrated in FIG. 28A, a pair of electrodesand the dielectrics have light-transmitting properties, and thus, thewhole storage capacitor has light-transmitting properties.

FIG. 28B illustrates an example of a structure of the storage capacitor,which is different from that in FIG. 28A.

FIG. 28B illustrates an example in which a storage capacitor is formedwith the capacitor wiring layer 430, and a stack of the capacitorelectrode 431 and the oxide semiconductor layer 405, in which the gateinsulating layer 402 is used as a dielectric. Here, the oxidesemiconductor layer 405 formed in contact with the capacitor electrode431 serves as one of electrodes of the storage capacitor. Note that theoxide semiconductor layer 405 is formed using the samelight-transmitting material in the same step as the source electrodelayer and the drain electrode layer of the transistor 460. Moreover,since the capacitor wiring layer 430 is formed using the samelight-transmitting material in the same step as the gate electrode layerof the transistor 460, the capacitor wiring layer 430 is arranged so asnot to overlap with a gate wiring layer of the transistor 460.

Although not illustrated, the capacitor electrode 431 is electricallyconnected to the pixel electrode layer 457.

Also in the storage capacitor illustrated in FIG. 28B, a pair ofelectrodes and the dielectrics have light-transmitting properties, andthus the whole storage capacitor has a light-transmitting property.

Each of the storage capacitors illustrated in FIGS. 28A and 28B haslight-transmitting properties; thus, sufficient capacitance and highaperture ratio can be obtained even when the size of a pixel isdecreased in order to realize higher definition of display images.

Note that this embodiment can be freely combined with any of the otherembodiments.

This application is based on Japanese Patent Application serial no.2009-196618 filed with Japan Patent Office on Aug. 27, 2009, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor layer;performing a first heat treatment on the oxide semiconductor layer;forming a source electrode layer and a drain electrode layer afterperforming the first heat treatment, wherein the source electrode layeris formed over and in contact with a first region of the oxidesemiconductor layer and the drain electrode layer is formed over and incontact with a second region of the oxide semiconductor layer; formingan inorganic insulating layer over and in contact with a third region ofthe oxide semiconductor layer, the source electrode layer, and the drainelectrode layer so that a resistance of the third region of the oxidesemiconductor layer is increased; and performing a second heat treatmentafter forming the inorganic insulating layer, wherein the third regionof the oxide semiconductor layer is between the first region of theoxide semiconductor layer and the second region of the oxidesemiconductor layer, and wherein a conductivity of the third region ofthe oxide semiconductor layer is lower than a conductivity of each ofthe first region and the second region of the oxide semiconductor layer.2. The method for manufacturing a semiconductor device according toclaim 1, wherein the first heat treatment on the oxide semiconductorlayer is performed under an atmosphere including nitrogen.
 3. The methodfor manufacturing a semiconductor device according to claim 1, whereinthe first heat treatment on the oxide semiconductor layer is performedat a temperature higher than or equal to 400° C.
 4. The method formanufacturing a semiconductor device according to claim 1, wherein thesecond heat treatment is performed on the inorganic insulating layer sothat oxygen in the inorganic insulating layer is supplied to the thirdregion of the oxide semiconductor layer.
 5. The method for manufacturinga semiconductor device according to claim 4, wherein the second heattreatment on the inorganic insulating layer is performed under anatmosphere including nitrogen.
 6. The method for manufacturing asemiconductor device according to claim 1, wherein the oxidesemiconductor layer comprises a crystal.
 7. The method for manufacturinga semiconductor device according to claim 1, wherein the oxidesemiconductor layer is a stacked film.
 8. The method for manufacturing asemiconductor device according to claim 1, wherein the oxidesemiconductor layer comprises indium, gallium, and zinc.
 9. The methodfor manufacturing a semiconductor device according to claim 1, whereinthe oxide semiconductor layer comprises indium, tin, and zinc.